PHP Warning: Cannot modify header information - headers already sent by (output started at ...)

What the message looks like

PHP Warning:  Cannot modify header information - headers already sent by (output started at /var/www/example.com/config.php:1) in /var/www/example.com/login.php on line 12

Common causes

• A UTF-8 BOM (hidden bytes) at the start of a PHP file, often saved by an editor, usually in config.php or a template.
• Whitespace or a blank line before <?php or after a closing ?> in an included file.
• echo, print or HTML output before header(), setcookie() or session_start().
• A PHP warning or notice printed to the page before the header call (display_errors on), which then causes this second error.
• A plugin or theme in a CMS that prints output early.

Check

1. Find the file named in the message and check its first bytes for a BOM
   head -c 3 /var/www/example.com/config.php | xxdCopy
2. Search PHP files for a BOM
   grep -rl $'\xEF\xBB\xBF' --include='*.php' /var/www/example.com | headCopy
3. Is there output after a closing tag at the end of the file?
   tail -c 30 /var/www/example.com/config.php | xxdCopy
4. Check the file for syntax problems
   php -l /var/www/example.com/config.phpCopy

Fix

• Open the file the message names and remove everything before <?php, including the BOM (save as UTF-8 without BOM).
• In files that contain only PHP, leave out the closing ?> so no trailing newline is sent.
• Move header(), setcookie() and session_start() above any output, or restructure so output happens last.
• If an earlier warning is the first output, fix it or set display_errors = Off in production and log errors instead.
• As a stopgap only, ob_start(); at the top buffers output, but fix the real cause.

FAQ

What does output started at config.php:1 mean?

The first byte was sent from line 1 of that file. That points to a BOM or whitespace before the opening tag.

Is it safe to ignore the warning?

No. The redirect, cookie or session that should have been set is lost, so logins and redirects can silently fail.