php ini output_handler

This interesting directive tells PHP to pass all output through a function before returning it to the requesting user.

For example, suppose you want to compress all output before returning it to the browser, a feature supported by all mainstream HTTP/1.1-compliant browsers. You can assign output_handler like so:

output_handler = "ob_gzhandler"

ob_gzhandler() is PHP's compression - handler function, located in PHP's output control library.