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<Output Control Functionsob_clean>
^Output Control FunctionsLast updated: Mon, 26 Oct 2026

flush

(PHP 4, PHP 5, PHP 7, PHP 8)

flush — Flush system output buffer

Description

function flush(): void

Flushes the system write buffers of PHP and the backend used by PHP (e.g.: CGI, a web server). In a command line environment flush() will attempt to flush the contents of the buffers only whereas in a web context headers and the contents of the buffers are flushed.

Note:

flush() may not be able to override the buffering scheme of the web server and it has no effect on any client-side buffering in the browser.

Note:

This function does not have any effect on user level output handlers such as those started by ob_start() or output_add_rewrite_var().

Warning

flush() can interfere with output handlers that set and send headers in a web context (e.g. ob_gzhandler()) by sending headers before these handlers can do so.

Parameters

This function has no parameters.

Return Values

No value is returned.

Changelog

Version Description
8.4.0 Flushing headers without a body will now succeed in FastCGI.

See Also

  • ob_flush() - Flush (send) the return value of the active output handler
  • ob_clean() - Clean (erase) the contents of the active output buffer
  • ob_end_flush() - Flush (send) the return value of the active output handler and turn the active output buffer off
  • ob_end_clean() - Clean (erase) the contents of the active output buffer and turn it off


<Output Control Functionsob_clean>
^Output Control FunctionsLast updated: Mon, 26 Oct 2026