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

How to fix "Headers already sent" error in PHP

The Issue:

Running a PHP script may produce errors stating that headers cannot be modified because output has already been sent. This typically occurs when header() or setcookie() calls are encountered after headers have been implicitly or explicitly sent.

Understanding the Problem:

HTTP headers must be sent before any output to the webserver. Functions that modify or send headers, such as header(), session_start(), and setcookie(), require headers to be sent beforehand.

Causes of Premature Output:

There are several reasons why premature output can occur:

• Unintentional output, such as whitespace or the UTF-8 Byte Order Mark (BOM) before PHP open tags (<?php).
• Intentional output such as print, echo, or raw HTML sections in a PHP file.

Locating the Source of Output:

The header() error message provides information about where the premature output occurred:

• "Output started at"`: Indicates the source of premature output.

Solutions:

• Ensure header() calls occur before any output is generated.
• Restructure code to avoid premature output from print, echo, etc.
• Consider templating schemes to separate output logic from processing.
• Verify that HTML sections do not occur before script logic that requires headers.
• Set text editors to save files as "UTF-8 (no BOM)" to avoid BOM issues.
• In rare cases, inspect files for whitespace or HTML characters after closing ?> tags.
• If the error source is unknown, check for issues with extensions or php.ini settings.

Output Buffering as a Workaround (Not Recommended):

Output buffering can be enabled to combine headers and output into a single pass. However, it is recommended to correct any code issues causing premature output instead of relying on buffering.

Additional Considerations:

• Use headers_sent() to check if it is still possible to send headers.
• Consider HTML <meta> tag or JavaScript redirect as fallbacks for header() failures.
• Apply the same guidelines for setcookie() and session_start(), which also require headers to be sent beforehand.

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