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PHP Output Control Functions

PHP Output Control Functions

PHP provides a set of functions that control what content is sent to the browser and when. This is referred to as output control.

Output can come from any of the following sources:

  • echo, print, printf, print_r... and other similar functions and statements
  • Notices, warnings and errors from PHP
  • Any content outside of the <?php ?> tags

PHP and the backend on which it is running may hold the output in a buffer before sending it to the user.

Note: The output control functions can create any number of output buffers. Output buffers catch output given by the program. Each new output buffer is placed on the top of a stack of output buffers, and any output it provides will be caught by the buffer below it. The output control functions handle only the topmost buffer, so the topmost buffer must be removed in order to control the buffers below it.


The PHP output control functions are part of the PHP core. No installation is required to use these functions.

Runtime Configuration

The behavior of output control functions is affected by settings in php.ini:

Name Default Description Version
output_buffering "0" Enables output buffering for all PHP files by default 4
output_handler NULL Set the name of the default function which handles the output of all output buffers 4
implicit_flush "0" Enables implicit flush, which causes output to be sent directly to the browser on each output statement 4
url_rewriter.tags "a=href,area=href, frame=src,form=,fieldset=" Indicates which HTML tags and attributes can be modified by the URL rewriter (the output_add_rewrite_var() function.) 4.3
url_rewriter.hosts The current value of $_SERVER['HTTP_HOST'] URL rewriting is only done on the server's own URLs by default. To allow for rewriting URLs of other websites, set the hostnames of the other websites here. 7.1

PHP Output Control Functions

Method Function
flush() Attempts to send content from the system's output buffer to the browser
ob_clean() Deletes all of the content from the topmost output buffer
ob_end_clean() Deletes the topmost output buffer and all of its contents
ob_end_flush() Deletes the topmost output buffer and outputs its contents
ob_flush() Outputs the contents of the topmost output buffer and clears the buffer
ob_get_clean() Returns all of the contents of the topmost output buffer and clears the buffer
ob_get_contents() Returns the contents of the topmost output buffer
ob_get_flush() Outputs and returns the contents of the topmost output buffer and then deletes the buffer
ob_get_length() Returns the number of bytes of data that are in the topmost output buffer
ob_get_level() Returns a number indicating how many output buffers are on the stack
ob_get_status() Returns information about the output buffers
ob_gzhandler() Used as a callback function for ob_start() to compress the contents of the buffer when sending it to the browser
ob_implicit_flush() Turns implicit flushing on or off
ob_list_handlers() Returns an array of callback function names that are being used by the topmost output buffer
ob_start() Creates a new output buffer and adds it to the top of the stack
output_add_rewrite_var() Used to append query string parameters to any URL in the output
output_reset_rewrite_vars() Removes all variables added by output_add_rewrite_var()