The full message is “Warning: Cannot modify header information - headers already sent by (output started at /path/file.php:12)”. It means PHP has already started sending text to the browser, and after that it can no longer change the headers (redirect, start a session, set a cookie, deliver a file). What matters is inside the brackets: the file and line where text started coming out. That is where the culprit is.
Reading the message
| Part of the message |
What it says |
| headers already sent by (output started at file:line) |
Where text started coming out. Start here. |
| ... in another-file.php on line N |
Where the code tried to change the header (a header(), session_start() or setcookie()). It is the victim, not the culprit. |
The causes, most common first
| 1 |
Spaces or blank lines outside the PHP. A blank line before <?php, or after a final ?>, is text sent out. It happens a lot in configuration files edited by hand, such as wp-config.php or a theme’s functions.php. The remedy is to delete the space and, in files that are pure PHP, stop using the closing ?>.
|
|
| 2 |
The “BOM”. Some editors (Notepad, for instance) save the file with three invisible bytes at the start. To catch them, in a Linux Terminal: grep -rl $'\xEF\xBB\xBF' --include=*.php .. Save the file again as “UTF-8 without BOM”.
|
|
| 3 |
An echo, print or HTML before the header(). In your own code, move the header change to the top, before any output.
|
|
The “trick” that postpones the problem
An ob_start() at the top of the code keeps the output in memory and sends it only at the end, so headers can still be changed. It fixes the symptom, not the cause, and hides the blank space that is still there. Here PHP ships with output_buffering off, so any byte before the header gives the error. Use ob_start() as a last resort, in your own code, and not as a fix for a third party’s file.
|
Always edit a copy. A mistake in a wp-config.php or a functions.php leaves the site blank. Save a copy of the file before touching it. If the site goes blank, see a blank page in PHP: how to see the error.
|
|
Have the message with the file and line and cannot see what is there? Send us the full message and the page address.
Open a support ticket
|
RECOMMENDED PRODUCT Web hosting with cPanel Domain and SSL included, daily backups and the panel you already know. from $6.60/mo (3-year plan, with coupon) See plans |
Angola
Brasil
Kenya
Moçambique
Nigeria
Portugal
South Africa
United Kingdom
United States
International
