ob_start（）

turn on output buffering

bool ob_start ([ callable $output_callback = NULL [, int $chunk_size = 0 [, int $flags = PHP_OUTPUT_HANDLER_STDFLAGS ]]] )

ob_get_contents( )

Return the contents of the output buffer

ob_end_flush( )

Flush(send) the output buffer and turn off output buffer.

ob_gzhandler

ob_start callback function to gzip output buffer
Note:
You cannot use both ob_gzhandler() and zlib.output_compression. Also note that using zlib.output_compression is preferred over ob_gzhandler().