Php ошибка headers already sent by

No output before sending headers!

Functions that send/modify HTTP headers must be invoked before any output is made.
summary ⇊
Otherwise the call fails:

Warning: Cannot modify header information — headers already sent (output started at script:line)

Some functions modifying the HTTP header are:

  • header / header_remove
  • session_start / session_regenerate_id
  • setcookie / setrawcookie

Output can be:

  • Unintentional:

    • Whitespace before <?php or after ?>
    • The UTF-8 Byte Order Mark specifically
    • Previous error messages or notices

  • Intentional:

    • print, echo and other functions producing output
    • Raw <html> sections prior <?php code.

Why does it happen?

To understand why headers must be sent before output it’s necessary
to look at a typical HTTP
response. PHP scripts mainly generate HTML content, but also pass a
set of HTTP/CGI headers to the webserver:

HTTP/1.1 200 OK
Powered-By: PHP/5.3.7
Vary: Accept-Encoding
Content-Type: text/html; charset=utf-8

<html><head><title>PHP page output page</title></head>
<body><h1>Content</h1> <p>Some more output follows...</p>
and <a href="/"> <img src=internal-icon-delayed> </a>

The page/output always follows the headers. PHP has to pass the
headers to the webserver first. It can only do that once.
After the double linebreak it can nevermore amend them.

When PHP receives the first output (print, echo, <html>) it will
flush all collected headers. Afterward it can send all the output
it wants. But sending further HTTP headers is impossible then.

How can you find out where the premature output occurred?

The header() warning contains all relevant information to
locate the problem cause:

Warning: Cannot modify header information — headers already sent by
(output started at /www/usr2345/htdocs/auth.php:52) in
/www/usr2345/htdocs/index.php on line 100

Here «line 100» refers to the script where the header() invocation failed.

The «output started at» note within the parenthesis is more significant.
It denominates the source of previous output. In this example, it’s auth.php
and line 52. That’s where you had to look for premature output.

Typical causes:

  1. Print, echo

    Intentional output from print and echo statements will terminate the opportunity to send HTTP headers. The application flow must be restructured to avoid that. Use functions
    and templating schemes. Ensure header() calls occur before messages
    are written out.

    Functions that produce output include

    • print, echo, printf, vprintf
    • trigger_error, ob_flush, ob_end_flush, var_dump, print_r
    • readfile, passthru, flush, imagepng, imagejpeg

    among others and user-defined functions.

  2. Raw HTML areas

    Unparsed HTML sections in a .php file are direct output as well.
    Script conditions that will trigger a header() call must be noted
    before any raw <html> blocks.

    <!DOCTYPE html>
<?php
    // Too late for headers already.

    Use a templating scheme to separate processing from output logic.

    • Place form processing code atop scripts.
    • Use temporary string variables to defer messages.
    • The actual output logic and intermixed HTML output should follow last.

  3. Whitespace before <?php for «script.php line 1» warnings

    If the warning refers to output inline 1, then it’s mostly
    leading whitespace, text or HTML before the opening <?php token.

     <?php
# There's a SINGLE space/newline before <? - Which already seals it.

    Similarly it can occur for appended scripts or script sections:

    ?>

<?php

    PHP actually eats up a single linebreak after close tags. But it won’t
    compensate multiple newlines or tabs or spaces shifted into such gaps.

  4. UTF-8 BOM

    Linebreaks and spaces alone can be a problem. But there are also «invisible»
    character sequences that can cause this. Most famously the
    UTF-8 BOM (Byte-Order-Mark)
    which isn’t displayed by most text editors. It’s the byte sequence EF BB BF, which is optional and redundant for UTF-8 encoded documents. PHP however has to treat it as raw output. It may show up as the characters  in the output (if the client interprets the document as Latin-1) or similar «garbage».

    In particular graphical editors and Java-based IDEs are oblivious to its
    presence. They don’t visualize it (obliged by the Unicode standard).
    Most programmer and console editors however do:

    joes editor showing UTF-8 BOM placeholder, and MC editor a dot

    There it’s easy to recognize the problem early on. Other editors may identify
    its presence in a file/settings menu (Notepad++ on Windows can identify and
    remedy the problem),
    Another option to inspect the BOMs presence is resorting to an hexeditor.
    On *nix systems hexdump is usually available,
    if not a graphical variant which simplifies auditing these and other issues:

    beav hexeditor showing utf-8 bom

    An easy fix is to set the text editor to save files as «UTF-8 (no BOM)»
    or similar to such nomenclature. Often newcomers otherwise resort to creating new files and just copy&pasting the previous code back in.

    Correction utilities

    There are also automated tools to examine and rewrite text files
    (sed/awk or recode).
    For PHP specifically there’s the phptags tag tidier.
    It rewrites close and open tags into long and short forms, but also easily
    fixes leading and trailing whitespace, Unicode and UTF-x BOM issues:

    phptags  --whitespace  *.php

    It’s safe to use on a whole include or project directory.

  5. Whitespace after ?>

    If the error source is mentioned as behind the
    closing ?>
    then this is where some whitespace or the raw text got written out.
    The PHP end marker does not terminate script execution at this point. Any text/space characters after it will be written out as page content
    still.

    It’s commonly advised, in particular to newcomers, that trailing ?> PHP
    close tags should be omitted. This eschews a small portion of these cases.
    (Quite commonly include()d scripts are the culprit.)

  6. Error source mentioned as «Unknown on line 0»

    It’s typically a PHP extension or php.ini setting if no error source
    is concretized.

    • It’s occasionally the gzip stream encoding setting
      or the ob_gzhandler.
    • But it could also be any doubly loaded extension= module
      generating an implicit PHP startup/warning message.

  7. Preceding error messages

    If another PHP statement or expression causes a warning message or
    notice being printed out, that also counts as premature output.

    In this case you need to eschew the error,
    delay the statement execution, or suppress the message with e.g.
    isset() or @() —
    when either doesn’t obstruct debugging later on.

No error message

If you have error_reporting or display_errors disabled per php.ini,
then no warning will show up. But ignoring errors won’t make the problem go
away. Headers still can’t be sent after premature output.

So when header("Location: ...") redirects silently fail it’s very
advisable to probe for warnings. Reenable them with two simple commands
atop the invocation script:

error_reporting(E_ALL);
ini_set("display_errors", 1);

Or set_error_handler("var_dump"); if all else fails.

Speaking of redirect headers, you should often use an idiom like
this for final code paths:

exit(header("Location: /finished.html"));

Preferably even a utility function, which prints a user message
in case of header() failures.

Output buffering as a workaround

PHPs output buffering
is a workaround to alleviate this issue. It often works reliably, but shouldn’t
substitute for proper application structuring and separating output from control
logic. Its actual purpose is minimizing chunked transfers to the webserver.

  1. The output_buffering=
    setting nevertheless can help.
    Configure it in the php.ini
    or via .htaccess
    or even .user.ini on
    modern FPM/FastCGI setups.
    Enabling it will allow PHP to buffer output instead of passing it to the webserver instantly. PHP thus can aggregate HTTP headers.

  2. It can likewise be engaged with a call to ob_start();
    atop the invocation script. Which however is less reliable for multiple reasons:

    • Even if <?php ob_start(); ?> starts the first script, whitespace or a
      BOM might get shuffled before, rendering it ineffective.

    • It can conceal whitespace for HTML output. But as soon as the application logic attempts to send binary content (a generated image for example),
      the buffered extraneous output becomes a problem. (Necessitating ob_clean()
      as a further workaround.)

    • The buffer is limited in size, and can easily overrun when left to defaults.
      And that’s not a rare occurrence either, difficult to track down
      when it happens.

Both approaches therefore may become unreliable — in particular when switching between
development setups and/or production servers. This is why output buffering is
widely considered just a crutch / strictly a workaround.

See also the basic usage example
in the manual, and for more pros and cons:

  • What is output buffering?
  • Why use output buffering in PHP?
  • Is using output buffering considered a bad practice?
  • Use case for output buffering as the correct solution to «headers already sent»

But it worked on the other server!?

If you didn’t get the headers warning before, then the output buffering
php.ini setting
has changed. It’s likely unconfigured on the current/new server.

Checking with headers_sent()

You can always use headers_sent() to probe if
it’s still possible to… send headers. Which is useful to conditionally print
info or apply other fallback logic.

if (headers_sent()) {
    die("Redirect failed. Please click on this link: <a href=...>");
}
else{
    exit(header("Location: /user.php"));
}

Useful fallback workarounds are:

  • HTML <meta> tag

    If your application is structurally hard to fix, then an easy (but
    somewhat unprofessional) way to allow redirects is injecting a HTML
    <meta> tag. A redirect can be achieved with:

     <meta http-equiv="Location" content="http://example.com/">

    Or with a short delay:

     <meta http-equiv="Refresh" content="2; url=../target.html">

    This leads to non-valid HTML when utilized past the <head> section.
    Most browsers still accept it.

  • JavaScript redirect

    As alternative a JavaScript redirect
    can be used for page redirects:

     <script> location.replace("target.html"); </script>

    While this is often more HTML compliant than the <meta> workaround,
    it incurs a reliance on JavaScript-capable clients.

Both approaches however make acceptable fallbacks when genuine HTTP header()
calls fail. Ideally you’d always combine this with a user-friendly message and
clickable link as last resort. (Which for instance is what the http_redirect()
PECL extension does.)

Why setcookie() and session_start() are also affected

Both setcookie() and session_start() need to send a Set-Cookie: HTTP header.
The same conditions therefore apply, and similar error messages will be generated
for premature output situations.

(Of course, they’re furthermore affected by disabled cookies in the browser
or even proxy issues. The session functionality obviously also depends on free
disk space and other php.ini settings, etc.)

Further links

  • Google provides a lengthy list of similar discussions.
  • And of course many specific cases have been covered on Stack Overflow as well.
  • The WordPress FAQ explains How do I solve the Headers already sent warning problem? in a generic manner.
  • Adobe Community: PHP development: why redirects don’t work (headers already sent)
  • Nucleus FAQ: What does «page headers already sent» mean?
  • One of the more thorough explanations is HTTP Headers and the PHP header() Function — A tutorial by NicholasSolutions (Internet Archive link).
    It covers HTTP in detail and gives a few guidelines for rewriting scripts.
Источник

Исправляем ошибку headers already sent by

От автора: в этом уроке мы рассмотрим, пожалуй, одну из наиболее распространенных ошибок, с которой сталкиваются при программировании на PHP. Эта ошибка возникает при работе с функциями, связанными с отправкой заголовков. При этом мы можем столкнуться с несколькими вариантами сообщений об ошибке, но причина у них будет одинакова – заголовки уже отправлены (headers already sent).

скачать исходникискачать урок

В видео версии урока вы найдете подробное описание вариантов возникновения данной ошибки. Здесь же мы сформулируем вкратце причины и способы решения данной ошибки.

Итак, ошибка эта может возникнуть в случаях, когда в коде вы используете одну из функций, связанных с отправкой заголовков: header(), session_start(), setcookie().

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Причина ошибки заключается в том, что перед данными функциями в коде где-то есть вывод в браузер. При этом выводом в браузер считается любой символ, даже пробел или перенос строки, которые можно увидеть в исходном коде.

Соответственно, решение проблемы заключается в том, что этот вывод нужно убрать. Варианты вывода могут быть следующими:

собственно вывод, который прописан в коде: это может быть пробел, перенос строки, HTML-код и т.д.

вывод в подключаемых файлах

сохранение документа в кодировке UTF-8 с BOM. Это наиболее неочевидная проблема и здесь стоит помнить, что при использовании юникода файл должен быть сохранен в кодировке UTF-8 без BOM.

На этом мы завершим текущий урок. Удачи и до новых встреч!

Источник

Most WordPress error messages give you an idea of what’s causing problems on your site. The “Warning: cannot modify header information – headers already sent by” error is no exception. If a PHP file cannot be executed due to a problem in its code, you’ll run into this message.

There are several potential causes for the “Cannot modify header information” error. Fortunately, the message itself will tell you which file is causing the problem. It even points to the line of code that contains the issue.

In this article, we’re going to discuss this error and its causes. Then, we’ll go over two ways that you can fix the problem. Let’s get to work!

What Causes the “Cannot Modify Header Information – Headers Already Sent By” Error

As we mentioned before, you’ll run into this error when one of your site’s .php files cannot be executed. WordPress relies on .php files, such as wp-config.php and functions.php, for its core functionality.

If there’s a problem within one of the .php files that your website needs to load, you’ll see an error message that looks like this:

Warning: Cannot modify header information - headers already sent by (output started at /home/public_html/wp-config.php:#) in /home/public_html/wp-includes/file-example.php on line 33

Fortunately, the “Cannot modify header information” error provides a lot of information that makes troubleshooting relatively straightforward. The message will point you toward two files – the first one contains the problem, which prevents the second one from executing.

At the end of the error message, you’ll see a section that says “line XX.” It shows the location of the specific code that’s causing the problem.

Usually, the problem in the PHP code is pretty easy to fix. Some common causes that can trigger the error message include:

  • Whitespaces before the <?phpsegment of the code or after the closing ?> tag
  • An HTML block before the PHP header function
  • print or echo statements added before the PHP header function
  • Problems with a plugin’s code

Fixing these types of errors requires you to be at least passingly comfortable with modifying PHP code. You won’t need to add any code yourself.

Still, you may need a bit of extra help identifying the problem. This is particularly true if the issue isn’t related to whitespaces or statements before the PHP header function.

The silver lining to seeing this error message- you already know which file is causing the problem and the line of code with the issue! 🤓 Learn how to fix it here 💪Click to Tweet

How To Troubleshoot the “Warning: Cannot Modify Header Information – Headers Already Sent By” Error (2 Methods)

There are two approaches to troubleshooting the “Cannot modify header information – headers already sent by” error. The first method doesn’t require you to exit the WordPress dashboard.

However, the second strategy uses FTP/SFTP if you can’t access the dashboard or use WordPress.

Let’s start with the first troubleshooting method.

1. Fix the Error With the Plugin/Theme Editor or Replace a Plugin

The first thing you need to do when you run into the “Cannot modify header information – headers already sent by” error is to open the file that’s causing the problem. Then, locate the line the message indicates.

For example, if you see an error that reads the following, it means you need to look inside your theme’s functions.php file:

Warning: Cannot modify header information - headers already sent by (output started at /home/public_html/wp-content/themes/twentytwentyone/functions.php:#) in /home/public_html/wp-includes/file-example.php on line 1

In this scenario, you can reach the source of the problem using the WordPress theme editor. To access it, go to Appearance > Theme Editor.

Once you’re in, use the menu to the right to select the file you need to access.

Theme Functions (functions.php) in the theme editor

Theme functions file (functions.php).

If you look closely, you’ll notice several whitespaces before the <?php tag. The error message itself points toward line number one. Therefore, this tells you that the whitespaces are the sources of the problem.

In this example, all you have to do is remove the whitespaces and click on Update File. Now try reloading your website, and the error should be gone.

You can apply the same process using the WordPress plugin editor (Plugins > Plugin Editor). This method is applicable if the error message points toward a faulty plugin file.

Alternatively, you may run into an error that indicates one of the files within your WordPress plugins directory. In this scenario, you can remove and reinstall that plugin. In most cases, that will take care of the issue for you.

However, keep in mind that you might lose that plugin’s configuration, depending on which tool you use. As such, you may need to set up the add-on again.

2. Edit the Problem File via FTP/SFTP

In some cases, the source of the “Cannot modify header information – headers already sent by” error won’t lie in a file that you can access using the WordPress theme or plugin editors. Alternatively, you might be using a non-WordPress site.

In these scenarios, your best option is to access the problem file using FTP/SFTP. To do so, you’ll need to use an FTP or SFTP client such as the FileZilla platform.

You’ll also need access to your website’s FTP/SFTP credentials. In most cases, you should be able to find them within your hosting panel.

If you use Kinsta, you can access MyKinsta, select your website under Sites and click on its Info tab.

Screenshot: SFTP details in MyKinsta.

SFTP details in MyKinsta.

Once you have the credentials, use your FTP or SFTP client to connect to your website. You’ll need to locate the site’s root folder. Usually, its name should be root, public_html, public, or your own site’s name.

Here’s a quick look at what the inside of a WordPress root folder looks like.

A look at the WordPress root folder

WordPress root folder.

Go ahead and locate the file that the “Cannot modify header information – headers already sent by” error indicates. For example, if the issue is public/wp-config.php, right-click on the file and select the View/Edit option.

Find the wp.config file in the root folder

Click on the wp.config file.

That option will open the selected file using your default text editor. Once the document is open, locate the problem by navigating to the line the error message pointed you toward.

Navigate to the line of the error message

Look for the line with the error message.

If you can’t spot the error, you might need to consult with someone who has experience working with PHP files. However, suppose you’re dealing with a whitespace issue or a statement before the PHP header. In that case, you should be able to fix the problem yourself.

Once you’re done, save the changes to the file and close the FTP/SFTP client. Try reaccessing your website, and the error should be gone.

Seeing this error message? 😥 This post has 2 guaranteed ways tod fix it 💪Click to Tweet

Summary

The “Warning: cannot modify header information – headers already sent by” error can be intimidating because it outputs a long message. However, that detailed error message makes this bug relatively simple to troubleshoot. Unlike other problems, this one is polite enough to tell you which file is causing it and which line of code you need to look into.

Depending on the file that’s causing the error, there are two ways that you can go about troubleshooting it:

  1. Fix the error using the plugin/theme editor or replace a plugin.
  2. Edit the problem file via an FTP/SFTP client.

Finding the source of this error is simple. However, fixing it can be a problem if you’re not familiar with PHP.

Still having issues fixing this error? Please share your experience with our community in the comments below!

Источник

Cannot modify header information - headers already sent

С этой ошибкой ко мне постоянно обращаются и спрашивают: «Где ошибка?«. Подобных писем за всё время я получил где-то штук 500, не меньше. Пора с ошибкой «Cannot modify header information — headers already sent» уже заканчивать. В этой статье я расскажу о причинах возникновения данной ошибки, а также о том, как её решить.

Если перевести данную ошибку на русский язык, то получится примерно следующее: «Нельзя изменить заголовок, поскольку они уже отправлены«. Что это за «заголовки«? Давайте разберёмся.

Когда сервер возвращает ответ клиенту, помимо тела (например, HTML-кода страницы), идут ещё и заголовки. В них содержится код ответа сервера, cookie, кодировка и множество других служебных параметров. Может ли PHP-скрипт отправить заголовок? Конечно, может. Для этого существует функция header().

Данная функция, например, постоянно используется при редиректе. Также данная функция регулярно используется при генерации изображении в PHP.

Также заголовки модифицируются при отправке cookie и при начале сессии (функция session_start()).

А теперь о том, почему же всё-таки возникает ошибка? Сервер всегда сначала отдаёт серверу заголовки, а потом тело. Если сервер уже вернул заголовки, потом пошло тело, и тут он встречает какой-нибудь session_start(). Оказывается горе-программист забыл отправить заголовки до начала тела, и теперь хочет догнать уже ушедший поезд.

Вот код с ошибкой «Cannot modify header information — headers already sent«:

<html>
<?php
  session_start(); // А давайте начнём сессию
?>

Разумеется, такой бред PHP не прощает. И надо было писать так:

<?php
  session_start(); // А давайте начнём сессию
?>
<html>

Вот этот скрипт уже не вызовет никаких ошибок, потому что сначала отправляются все заголовки, а уже потом идёт генерация тела ответа сервера.

Другой пример кода с ошибкой:

<?php
  echo "Hello!"; // Что-нибудь выведем
  session_start(); // А давайте начнём сессию
?>

То же самое, почему-то сначала выводится тело (либо его кусок), а потом вспомнили, что ещё и надо заголовки модифицировать.

Как будет правильно переписать данный код, подумайте сами.

Ещё пример:

<?php
  $error = true; // Были ли ошибки?
  if ($error) echo "Произошла ошибка";
  header("Location: ".$_SERVER["HTTP_REFERER"]); // Делаем редирект обратно
  exit;
?>

Когда у автора такого кода, ничего не получается, он удивляется от этой ошибки и говорит: «Очень странное совпадение, когда операция проходит успешно, всё хорошо, а когда какая-то ошибка, мне сообщают Cannot modify header information — headers already sent». Не дословно, но смысл именно в этом.

Проблема та же самая, и правильно писать так:

<?php
  $error = true; // Были ли ошибки?
  if ($error) echo "Произошла ошибка";
  else header("Location: ".$_SERVER["HTTP_REFERER"]); // Делаем редирект обратно
  exit;
?>

Есть и труднозаметные ошибки:

 <?php
  header("Location: ".$_SERVER["HTTP_REFERER"]); // Делаем редирект обратно
  exit;
?>

Ошибка в данном коде возникает из-за пробела, который присутствует перед <?php. Пробел — это обычный символ, и он является частью тела ответа. И когда сервер его видит, он делает вывод о том, что заголовков больше не будет и пора выводить тело.

Бывают и следующие ошибки, имеющие всё ту же природу. Допустим есть файл a.html:

<?php echo "Hello"; ?>

Далее есть другой файл с таким кодом:

<?php
  require_once "a.html";
  header("Location: ".$_SERVER["HTTP_REFERER"]); // Делаем редирект обратно
  exit;
?>

И человек искренне удивляется, откуда ошибка, если он ничего не выводил? Поэтому смотреть надо не конкретно 1 файл, а все файлы, которые подключаются в нём. И в тех, что подключаются у подключаемых, тоже надо смотреть, чтобы не было вывода.

И последний момент, но уже более сложный. Оказывается, что иногда эта ошибка происходит и при правильном коде. Тогда всё дело в кодировке. Убедитесь, что кодировка файла «UTF-8 без BOM«, причём именно «без BOM«, а не просто «UTF-8«. Поскольку BOM — это байты, идущие в самом начале файла, и они являются выводом.

Очень надеюсь, что данная статья поможет решить абсолютно все проблемы, связанные с ошибкой «Cannot modify header information — headers already sent«, поскольку я постарался осветить все возникающие проблемы. А дальше надо включить голову, и подумать, а что в Вашем коде не так?

  • Создано 24.12.2012 06:35:29


  • Михаил Русаков

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Introduction

Interpreting error messages in WordPress can be a time-consuming process. Finding an adequate solution to the problem is equally as difficult.

To keep your website running like clockwork, you need to prevent long periods of downtime.

Find out how to fix the “Cannot Modify Header Information” error in WordPress and get your site up and running quickly.

Introductory image to WordPress Cannot Modify Header Information error.

What Does “Cannot Modify Header Information – Headers Already Sent By” Mean?

The “Cannot Modify Header Information – Headers Already Sent By” error indicates that a .php file cannot execute because an output is being sent before calling an HTTP header. Headers always need to precede the output.

The most common causes of the «Cannot Modify Header Information» error are:

  • Whitespaces before the opening <?php token.
  • Whitespaces after the closing ?> tag (if one is present).
  • An HTML block is in front of the header within the .php file.
  • Statements that produce output, such as print or echo, are being called before the header.
  • Issues with an installed plugin.

The warning message tells you which file or files you need to review to fix the issue.

Example of "Cannot Modify Header Information" error in WordPress.

In this example, the warning message indicates that the plugable.php file cannot execute due to an issue within the wp-config.php file.

Warning: Cannot modify header information - headers already sent by (output started at /home/public_html/example.com/wp-config.php:33) in /home/public_html/example.com/wp-includes/plugable.php on line 1063

You need to access and check the code within the wp-config.php file.

The message also provides the path to the file /home/public_html/example.com/wp-config.php. The number 33 points out the exact line within the code causing the error.

The file path and the code line number are an excellent starting point when troubleshooting.

Corrupted PHP File

Use cPanel or an FTP client to access the server the website is located on. It is recommended to back up files before editing them.

The examples in this guide are presented using cPanel.

Edit the File

1. Locate the file by using the path from the warning message. You can also use the search bar at the top of the cPanel dashboard.

2. Select the file and click Edit.

How to edit WordPress files using cPanel.

3. Find the code line number specified in the warning message. Use the Go To Line option to locate lines in lengthy files quickly.

4. Remove any whitespaces preceding the opening <?php token.

Locate the code line causing the error and remove all whitespaces.

5. If the file contains a ?> closing tag, ensure that there are no whitespaces after the tag.

6. Click the Save Changes tab once you remove all redundant whitespaces.

Save changes to the PHP file in cPanel.

Once you reload your webpage, the error message should disappear.

Move the Header Statement

Raw HTML elements in a .php file are rendered as a direct output. If an HTML element is placed before a header call, it can cause the “Cannot Modify Header Information – Headers Already Sent By” error.

To fix the error, place the HTML block after the header statement.

An HTML element preventing a header statement call.

Functions that produce output such as vprintf, printf, echo, flush, and print statements must follow HTTP header calls.

Review the .php file specified in the error message and correct the code. Always position header calls before the output producing elements and functions.

Replace the File

If you cannot locate the corrupt file or you are hesitant to edit the code, you can replace the entire file instead.

1. Download the latest WordPress version.

2. Use an FTP client or cPanel to upload the new version of the file to your server.

3. Overwrite the existing corrupt file with the new version.

Overwrite a corrupt PHP file in WordPress.

4. Reload the previously unavailable webpage.

You can upload and replace entire WordPress folders if the warning displays issues in multiple files.

Find the Plugin that Causes the Error

A faulty plugin can cause the “Cannot Modify Header Information” error.

1. Access your WordPress dashboard.

2. Deactivate and then Activate each plugin, in turn, to determine if one of them is causing the issue.

3. Refresh the affected webpage after every change you make.

Where to deactivate plugins in WordPress.

Focus on the recently installed or updated plugins. The error will reappear once you turn on the failing plugin.

Gather as much data as you can during this process and inform the plugin developers of the issues you encountered.

Conclusion

By following the instructions in this guide, you have successfully identified and resolved the “Cannot Modify Header Information” error.

Warning messages that flag similar issues are not uncommon in WordPress. The presented solutions are applicable in most cases with related underlying causes.

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Image credits : wikipedia commons

This PHP header error results from extra empty spaces that are not suppose to be within your files. You could have unknowingly created them when you are modifying WordPress or your Theme files. You will see something similar to the following error message.

Warning: Cannot modify header information — Header already sent by (Output started at /blog/wp-config.php:34)

Locating the error

We can troubleshoot this issue by looking at the file which the error message is pointing to. Looking at the above sample error, we can see that it points to wp-config.php. 

This error usually happens in wp-config.php or functions.php, because both files are most frequently edited by users. If you happen to be editing a theme file other than functions.php, then it’s highly possible that the error is from the file that you have just edited. 

Therefore, it’s always good practice to keep a list of files that you have worked on. In programming, we call this file a changelog.

Troubleshooting

You will need to use your FTP program to edit the file on your server.

  1. Use your FTP program to login to your web server.
  2. Navigate to your file, in the example, it’s the wp-config.php.
  3. Open up wp-config.php file.
  4. Look for <?php at the very beginning of the file. Remove any empty space before <?php
  5. The <?php which is the PHP opening tag should be at the very first line of the file within wp-config.php
  6. Look for any ?> sign at the end of the file, it there is any, you will need to remove any empty space after it.
  7. If you cannot find any ?> sign at the end of the file, you will not need to do anything. Just to play safe, you can remove empty lines.
  8. If your error points to functions.php, you can carry out the same procedure.

Error caused by plugin

If you are unable to locate the error, and unable to fix it. There is a high chance that it’s coming from a plugin. You can disable all plugins to see if it fixes your issue. If indeed it’s cause by a plugin, you can start finding it by enabling one plugin at a time, and re-loading your website. 

After you have found the plugin that’s causing this issue, you will need to disable it and report this issue to the plugin developer.

External Resources

  1. How to fix «Headers already sent» error in PHP (stackoverflow)
  2. Cannot modify header information — headers already sent (wordpress.org)

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Last updated on October 18, 2020

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This is probably one of the most common PHP errors that you will come across as you struggle to come to grips with the intricate world of web development:

Cannot modify header information – headers already sent in (file location).

To sum this error up – It is basically telling you that you cannot output data before you attempt to modify header information. A quick example:

<?php

//Example output using echo.
echo 'Example';

//Attempt to redirect.
header('Location: index.php');

The code above will cause the error in question, simply because we attempted to change our header information after we had already printed out the string “Example” to the browser.

Common causes of this particular error are as follows:

  1. Attempting to modify header information after output has already been sent to the client (as seen in the code example above).
  2. Random whitespace and newline characters above the opening PHP tag.
  3. Whitespace after closing PHP tags in files that have been included at the top of the script.
  4. PHP errors causing error messages to be printed out onto the screen before the headers are modified.
  5. Attempting to modify header information after HTML has already been displayed.

An example of whitespace causing the “headers already sent” error:

<?php

header('Location: index.php');

Here, we can see that there is a blank line before our opening PHP tag. This blank line is interpreted as output by the server, resulting in the “headers already sent” error.

An example of HTML causing issues:

<h1>Example</h1>
<?php

header('Location: test.php');

Our HTML is interpreted as being output, which prevents us from carrying out the intended redirect.

Debugging.

To debug this issue, you need to learn how to understand the warning message that PHP displays. Lets take the following example:

Warning: Cannot modify header information – headers already sent by (output started at /www/my-page.php:12) in /www/my-page.php on line 67

Here, we can see that PHP is telling us what the problem is being caused by LINE 12 in my-page.php. It is also telling us that our attempt to modify the header information failed on LINE 67 on my-page.php

Basically, the output on LINE 12 prevented our attempt to modify the header information on LINE 67.

Modifying Header Info.

There are a number of PHP functions that allow us to modify the HTTP header that the client receives. Popular examples include the functions header, session_start and setcookie. When using these functions, you MUST make sure that no output has been sent to the client beforehand.

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Ошибка «Невозможно изменить информацию заголовка» означает, что вы правили файлы (скорее всего, wp-config.php) вручную. И правили некорректно. Необходимо сохранять файлы в кодировке UTF-8 без метки BOM (byte order mark).

Имя файла, приводящего к ошибке, и номер строки указаны в «output started at». Например:

Warning: Cannot modify header information — headers already sent by (output started at /home/user/site.ru/public_html/wp-config.php:1) in /home/user/site.ru/public_html/wp-includes/pluggable.php on line 934

означает, что проблему вызывает 1-я строка файла wp-config.php.

  1. Убедитесь, что перед первой строкой <?php и после последней ?> нет пустых строк.
  2. Избегайте править файлы в Блокноте. Используйте «программистские» редакторы вроде PSpad, Notepad++ и им подобные, в которых метка BOM отключается. В Notepad++ для этого нужно выбрать в меню «Кодировки» пункт «Преобразовать в UTF-8 без BOM».

Также к предупреждению Cannot modify header information приводит вывод любых других сообщений об ошибках, предупреждений и нотаций php, предшествующих выводу заголовков.

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