Which PHP (opcode) cache one should use and why?
Solution 1:
The products you list serve different purposes.
OPCode caches
There are many PHP Accelerators (OPCaches) as seen on this Wikipedia list. As is common with open source products, they are all fairly similar. XCache is the lighttp PHP accelerator, and is the default choice when you are running that HTTPd. It works well with Apache as well, however APC seems to be slightly more "plays well with others" socially speaking, being officially supported as part of PHP, and is released in-step with the official PHP distribution.
I abandoned usign eAccelerator due to its slowing development, and lagging against the releases of PHP, and the official blessed status APC offers with similar performance.
These products typically are drop in; no code change instant performance boost. With large codebases (Drupal, Wordpress) the performance can be up to 3x better while lowering response time and memory usage.
Data Caching
Memcache is a slightly different product -- you might think of it as a lightweight key value system that can be scaled to multiple servers. Software has to be enhanced to support Memcache, and it solves certain problems better than others. If you had a list of realtime stock values on your website, you might use Memcache to keep a resident list of the current value that is displayed accross your website. You might use it to store session data for short term reuse. You wouldn't use it for other things such as full-page caches, or as a replacement for MySQL.
There are also Wordpress addons such as WP-Super-Cache that can drastically improve Wordpress' performance (infact, WP-Super-Cache can rival static HTML based sites in many cases)
In summary -- I would highly recommend APC if you want a "set it and forget it, well supported product".
Solution 2:
A good answer was posted on stackoverflow which answers your question nicely.
https://stackoverflow.com/questions/28716/which-php-opcode-cacher-should-i-use-to-improve-performance
Solution 3:
APC will be built into PHP6, so it's a logical choice. I use it, and the performance boost is amazing. If you need to cache something other than opcodes (i.e. db query results), you can use APC for that, too, but it's not possible to share APC caches between multiple servers. If you only need to cache on a single server, APC is great. If you need to scale out to multiple servers, and want to share a cache between them, memcached is your man.
One thing I would do, though, is create a wrapper class for any (non-opcode) caching you do. That way you can swap out the caching engine without changing your code.
Solution 4:
Just to note that things has changed a little bit and it seems that APC will not be included in PHP 6 core.
APC has slow development and it looks like it will never be PHP 5.5 compatible. Because of that, looks like guys from PHP will be setting Zend OPCache opcode cache extension as the PHP CORE extension. You care read more here http://wiki.php.net/rfc/optimizerplus.
Important note: Zend OPCache does not have user data cache like APC, so if you need user data cache you can you use it together with Memcache.
Solution 5:
If running PHP version at least 5.50, OpCache is your best bet (PHP / PECL native library). It should come pre-compiled if installing from binary.
http://php.net/manual/en/book.opcache.php
If running PHP version prior to 5.5, APC (PHP / PECL native OpCode cache) would be the simplest choice, although it's considered unmaintained and dead:
http://php.net/manual/en/book.apc.php
Using the native OpCache functionality of PHP should save you the trouble of maintaining 3rd party libraries.