Anyone who runs a dedicated server for web hosting will tell you that a great way to decrease the load on your server and decrease the page load time is to use a PHP Cache such as APC or eAccelerator. While the largest noticeable improvements are for those site that receive a lot of traffic or are under heavy load, any site, large or small can see benefit from a PHP cache. That said, in addition to the two caches mentioned above, a new player has recently entered the market: XCache.
I first started using APC about 2 years ago when the load on one of my servers was high enough that it was affecting load times and was costing me user traffic. I chose APC over eAccelerator because it was a bit easier to install (at the time) and because APC had a reputation for being a bit faster than eAccelerator. Shortly there after I noticed my httpd processes segfaulting and a bit of research also showed that APC had a bit of a record for instability under heavy load. With that in mind, I took the slight performance hit and installed eAccelerator (which is still way faster than using nothing at all).
Up until today, I was still using eAccelerator on all of my servers. However, a post on the vBulletin.com forums prompted me to give XCache, the new PHP accelerator from the maker of lighttpd, a try. I’ve got to say, while I’ve only been using it for about 6 hours at this point, it blows eAccelerator out of the water, especially once you enable multiple caches (which benefits SMP systems).
If you’re interested in some benchmarks of XCache, eAccelerator, APC, etc. then checkout the Five Opcode Cache Comparison on PHP on Fire.