Take note that if you change zlib output compression setting in between ob_start and ob_end_clean or ob_end_flush, you will get an error: ob_end_flush() failed to delete buffer zlib output compressionExample:<?phpob_start();$output = ob_get_contents();ini_set('zlib.output_compression', '1');ob_end_clean();?>ob_end_clean(); in this example will throw the error.