Goal: A php accelerator should be integrated as a standard feature of all web appliances.
All LAMP based TurnKey web appliances should come with a php accelerator pre-integrated.
A PHP accelerator typically reduces server load and increases the speed of PHP code anywhere from 2-10 times, depending on factors such as the inherent execution time of the PHP application and the percentage of source code actually executed on a given request
The leading candidate is xcache, for which a package already exists in Ubuntu hardy universe (php5-xcache)!
Its written by the extremely skilled programmer who is also behind lighttpd, and he wrote XCache after studying eAccelerator and APC.
The other alternatives eaccelerator and APC are a bit more problematic.
eAccelerator is very good (also written by a technical genius) but has a licensing problem. Also, from what I've been reading XCache performs somewhat better.
APC is supposedly the future (supposed to be integrated into PHP6 by default), and written by the official php team but I've been reading quite a few reports regarding stability issues (segfaults and such under
high load), and there is also the conflict of interest with a Zend commercial offering that makes me suspect that is the reason APC isn't what it could/should be.