Why am I getting the PHP warning 'Cannot modify header information - headers already sent'?

What does this PHP error mean: Warning: Cannot modify header information - headers already sent

Explanation:

When you encounter this warning, it indicates that your PHP script has attempted to modify HTTP headers after they have already been sent to the client. This typically occurs when you attempt to send headers after sending other output to the browser, such as HTML markup or whitespace.

Cause:

This error is typically caused by one of the following:

• Outputting content before calling header-related functions (e.g., header(), setcookie())
• Having whitespace or newlines before the opening
• Having multiple blocks in a file with spaces between them
• Using an editor that inserts Byte Order Marks (BOMs) into UTF-8 encoded files

Solution:

To resolve this issue, ensure that you send header-related commands before any output is sent to the browser. This includes removing any whitespace or newlines before the opening blocks, and checking your editor settings to prevent BOMs from being inserted into your code.

If you are unable to determine the source of the output, you can use debug tools such as Xdebug or print_r() statements to identify the specific line in your code that is causing the issue.

Related Questions:

• [Headers already sent by PHP](https://www.php.net/manual/en/function.headers-sent.php)
• [All PHP "Headers already sent" Questions on Stackoverflow](https://stackoverflow.com/questions/tagged/php headers-already-sent)
• [Byte Order Mark](https://en.wikipedia.org/wiki/Byte_order_mark)
• [What PHP Functions Create Output?](https://www.php.net/manual/en/features.output.php)

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