Hello Google begans to rank sites with the speed each site loads.
So how can i increase the speed of a php mysql site ? I am already Gzip compression
Use memcache(d) to cache the result of common queries, then look up results against memcache first, it's far quicker than making repeated lookups to a database.
You'll need to rewrite some of your logic to have a "if value is not in memcache, then do the query, and store the data in the cache system" kind of algorithm.
Also consider using a reverse proxy to cache generated pages, fragments and images. Load images and media from a seperate cookie-less domain, from a server running something that can serve faster than apache. Nginx and lighttpd are good options for media servers.
There are many options... caching PHP pages is a good way to give most people a good experience.
http://en.wikipedia.org/wiki/List_of_PHP_accelerators
Is this a site written from scratch, or something like Wordpress, Joomla, etc? If you're running a CMS then there's very specific advice to give based on platform.