PHP has a built-in output compression system that will allow it to make the web pages it sends to your vistors smaller.  Obviously, less data to send means less time to send those pages and your pages are served faster!  To try out output buffering, you need to adjust two settings in the php.ini file:

output_buffering = Off   (change the Off to On)
output_handler =   (switch to output_handler = ob_gzhandler )

Run the phpspeed benchmark before and after changing these settings to determine how helpful they may be for your server.