Note: mode was added in PHP 4.0.5.
ob_gzhandler() is intended to be used as a callback function for ob_start() to help facilitate sending gz-encoded data to web browsers that support compressed web pages. Before ob_gzhandler() actually sends compressed data, it determines what type of content encoding the browser will accept ("gzip", "deflate" or none at all) and will return it's output accordingly. All browsers are supported since it's up to the browser to send the correct header saying that it accepts compressed web pages.
See also ob_start() and ob_end_flush().