Why Am I Getting the 'Headers Already Sent' Error in PHP and How Can I Fix It?

How to Troubleshoot "Headers already sent" Error in PHP

When executing PHP scripts, you may encounter the following error:

Warning: Cannot modify header information - headers already sent by (output started at /some/file.php:12) in /some/file.php on line 23

This error occurs when headers are attempted to be sent after output has already been sent to the client. Here's why this happens and how to resolve it:

Premature Output

Functions like header(), header_remove(), session_start(), and setcookie() require headers to be sent before any output is generated. If output is generated before these functions are called, the warning will be triggered.

Output can occur unintentionally due to:

• Whitespace or extra newlines before or after PHP opening/closing tags
• UTF-8 BOM (Byte Order Mark)
• Previous error messages or notices

Intentionally, output can be generated by:

• print, echo, and other functions that produce output
• Raw HTML sections prior to PHP code

Identifying the Output Cause

The error message provides the line number and file where the premature output occurred and where header() was invoked. Look for the line mentioned in the "output started at" portion of the error message to determine where the output was generated.

Resolving the Issue

1. Remove Premature Output:

Ensure that there is no premature output before header() calls. This includes removing extra whitespace, BOMs, or intentional output like echo statements.

2. Use Output Buffering:

Output buffering can help alleviate this issue by delaying the output from being sent to the client. Enable output buffering through the output_buffering configuration setting in php.ini, .htaccess, or .user.ini.

3. Check with headers_sent():

Use headers_sent() to check if it's possible to send headers before executing sensitive actions. If headers_sent() returns true, use alternative methods such as HTML meta tags or JavaScript redirects.

4. Separate Control and Output Logic:

Refactor code to separate control logic and output generation. This helps prevent premature output by ensuring that header calls are made before any output is displayed.

5. Fix BOMs with Editor or Tools:

BOMs can be present in text files and can lead to premature output. Use text editors with BOM detection or tools like phptags to correct BOM issues.

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