Why Am I Getting a 'Headers Already Sent' Error in PHP?

Headers Already Sent Error in PHP

When running PHP scripts, you may encounter errors like:

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

This occurs when HTTP headers are attempted to be sent/modified after any output has been generated. Functions like header(), setcookie(), and session_start() require headers to be sent before any output is made.

Causes of Headers Already Sent Errors

Unintentional Output:

• Whitespace before
• UTF-8 Byte Order Mark (BOM)
• Previous error messages or notices

Intentional Output:

• Functions like print, echo, or others producing output
• Raw HTML sections

Identifying the Source of Output

The error message usually includes the line where the header function was invoked (e.g., line 23) and the source of premature output (e.g., line 12).

Solutions

Avoid Premature Output:

• Use functions and templating schemes to restructure application flow.
• Ensure header() calls are made before writing messages or rendering output.

Fix Specific Causes:

• Print/Echo: Use functions like trigger_error, ob_flush, or var_dump to manage output.
• Raw HTML: Place form processing and script conditions before HTML sections.
• Whitespace: Remove leading whitespace or use long PHP tags () instead of shorthand (
• UTF-8 BOM: Set your text editor to save files as "UTF-8 (no BOM)" or use a hex editor to remove BOMs.
• Trailing Whitespace after ?>: Omit trailing PHP close tags.
• Other: Check for PHP extensions or php.ini settings that may cause implicit output.
• Error Messages: Eschew errors, delay their execution, or suppress them using isset() or @().