PHP Version 5.1.4 here. What Bob said is correct even in this version (newest at the time of writing). You can't enable zlib.output_compression via ini_set(). You have to use php.ini.PHP Version 5.1.4 here. What Bob said is correct even in this version (newest at the time of writing). You can't enable zlib.output_compression via ini_set(). You have to use php.ini.