Warning: session_start(): Session cannot be started after headers have already been sent
session_start() ran after output was sent, so PHP can’t set the session cookie.
Exact message you may see
Session cannot be started after headers have already been sentsession_start(): Cannot start session
Seen on:
PHP
Meaning
Sessions need a cookie header, and headers must be sent before any output. A BOM, whitespace before <?php, or an echo/warning earlier in the request causes it.
Common causes
- HTML or whitespace before session_start()
- UTF-8 BOM in an included file
- A PHP notice printed earlier
- session_start() called inside a template
⚡ Quick fix
- Call session_start() at the very top of the entry script
- Remove whitespace/BOM before
<?phpand closing?>tags - Turn display_errors off in production
Detailed fix by platform
PHP
- Move
session_start()into your bootstrap file included first by every page
How to diagnose
- Output start — The warning names the file/line where output started
- Order — Is session_start() the first thing that runs?
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Last updated 14 Oct 2026