Why Am I Getting the 'Headers Already Sent' Warning in PHP?-PHP Tutorial-php.cn

Why Am I Getting the 'Headers Already Sent' Warning in PHP?

Patricia Arquette

Release： 2024-12-26 08:58:09

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Why Am I Getting the

Warning: Cannot Modify Header Information - Headers Already Sent

This warning can be seen in PHP scripts when attempting to modify the HTTP header after it's already been sent to the client. By default, PHP sends the header when any output is sent to the web server.

To resolve this warning, relocate the header sending code before any PHP output is generated. Check every object in the code to detect any hidden PHP output in functions, methods, and loops.

For example, consider this template:

<html>
  <?php session_start(); ?>
  <head><title>My Page</title>
</html>

Copy after login

This code will cause a header already sent warning since the session_start() function sends a session cookie before the element is sent. To fix this, move the session_start() to the top of the file.

It's also important to note that empty spaces, newlines, or other invisible characters before the opening or closing PHP tags can trigger this warning.

If your code utilizes multiple PHP blocks, ensure there are no spaces between them. Byte Order Marks (BOMs) in the code's encoding can also cause the issue.

Here are some related references for further investigation:

[Headers already sent by PHP](https://www.php.net/manual/en/function.headers-already-sent.php)
[All PHP "Headers already sent" Questions on Stack Overflow](https://stackoverflow.com/search?q=headers already sent)
[Byte Order Mark](https://en.wikipedia.org/wiki/Byte_order_mark)
[What PHP Functions Create Output?](https://stackoverflow.com/questions/263249/what-php-functions-create-output)

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