PHP

Headers already sent (PHP)

Something already sent output to the browser, so PHP can't set headers/cookies/redirects anymore.

What it looks like

Warning: Cannot modify header information - headers already sent by (output started at /var/www/app/config.php:1) in /var/www/app/login.php on line 15

What does "Headers already sent (PHP)" mean?

A warning raised when code calls header(), setcookie() or session_start() after output has begun. The key part is in brackets: “output started at file:line” names exactly what sent the first byte.

What causes "Headers already sent (PHP)"?

• Whitespace or text before the opening PHP tag (or after the closing one).
• A BOM at the start of the file.
• An echo/var_dump before header()/setcookie().

How do I confirm the cause?

1. Open the file and line given after “output started at” — that is where the stray output is.
2. Look for blank lines or spaces before <?php, or after a closing ?> at the end of an included file.
3. Check the file for a UTF-8 BOM (an invisible marker some editors add); re-save it as UTF-8 without BOM.

How do I fix "Headers already sent (PHP)"?

• Remove any output (incl. blank lines/BOM) before the header call.
• Don't add a closing PHP tag at the end of pure-PHP files.
• The error names the file:line that sent output first.

How do I stop it happening again?

• Leave out the closing ?> tag at the end of files that contain only PHP.
• Set your editor to save files as UTF-8 without BOM.

Got a different error? Paste it into the Explain This Error tool → Identifies the family and the fix — runs locally, nothing uploaded.

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