In the end I went with eAccelerator - the speed boost, the smaller memory footprint and the fact that is was very easy to install swayed me. It also has a nice web-based front end to clear the cache and provide some stats.
The fact that its not maintained anymore is not an issue for me - it works, and that's all I care about. In the future, if it breaks PHP6 (or whatever), then I'll re-evaluate my decision and probably go with APC simply because its been adopted by the PHP developers (so should be even easier to install)
I think APC is the way to go unless you are using Zend Optimizer on the site. APC is incompatible with Zend Optimizer so in that case you will need to go with something like eAccelerator.
I tested eAccelerator and XCache with Apache, Lighttp and Nginx with a Wordpress site. eAccelerator wins every time.
The bad thing is only the missing packages for Debian and Ubuntu. After a PHP update often the server doesn't work anymore if the eAccelerator modules are not recompiled.
eAccelerator last RC is from 2009/07/15 (0.9.6 rc1) with support for PHP 5.3
In all tests I have seen, eAccelerator performs faster than any other cache out there and uses less memeory to do so. It comes with a nifty script to view cache utilisation and clear the cache etc. eAccelerator is compatible with xdebug and Zend Optimizer.
APC is being included in PHP because it is being maintained by the PHP developers. It performs very well, but not as good as eAccelerator. And it has compatability issues with Zend Optimizer.
Xcache was made by the developers of lighttpd, benchmarks show it performs similiarly to eAccelerator, and faster than APC.
So which is the best?
APC = Great if you want an easy cache that will always work with PHP, no fuss.
eAccelerator = If you have time to maintain it, keep it up todate and understand how it works, it will perform faster. Long term support not as certain as APC because APC is done by the PHP devs.
Even both eacceleator and xcache perform quite well during moderate loads, APC maintains its stability under serious request intensity. If we're talking about a few hundred requests/sec here, you'll not feel the difference. But if you're trying to respond more, definetely stick with APC. Especially if your application has overly dynamic characteristics which will likely cause locking issues under such loads. http://www.ipsure.com/blog/2011/eaccelerator-as-zend-extension-high-load-averages-issue/ may help.
I always used APC with php 5.1 and 5.2, but I had a lot of (random) errors using APC with php 5.3: Strange blank pages, random out-of-memory errors. They all disappeared when I disabled APC. But that was no option, as it is running a high-volume website.
So I tried eaccelerator. So far it has been rock solid and the speed increase is even bigger than with APC. The APC guys really need to spend some time on bugfixing.
