This option is used to enable buffering/compression of HTML output if the user's browser supports it. Default value is ON. This option will be ignored in the following scenarios:
If php.ini has zlib.output_compression enabled.
If php.ini has output_handler set to a handler.
If PHP does not include the zlib extension (PHP 4.3.0 and later include zlib extension by default).
You can check the loaded modules in your PHP by running "php -m" on the command line, or by using php_info() command in a php script.