Well let me go ahead and tell you. Of course, this is all just my opinion and your milage may vary.
First, I will discuss the role of PHP accelerators (Opcode cache) in server tuning and scalability briefly. These tools will not enable your server to handle much more traffic in most scenarios. In some, the additional overhead of the PHP caching will even cause more load, others gain a marginal improvement by getting requests served a bit more quickly and having fewer concurrent connections. But they will not significantly raise your concurrent user limitations, as in a LAMP stack your bottleneck is usually at the database.
The way these PHP accelerators work is by caching the compiled bytecode of your human-readable PHP. Normally, your PHP code is compiled and then executed at runtime but these tools cache the compiled code, saving the expense of compiling it and thusly generally save you a bit of CPU at the cost of some increased memory usage.
So what PHP acceleration can do is make your PHP execute more quickly, and execute in roughly half the time. But it’s important to understand just what it’s accelerating, because your PHP execution is typically not what most influences the perception of speed to the user. To the user it’s a combination of page generation time, network latency, and page rendering time. These php caching tools may influence the page generation time but as I’ve already said, the database is usually the key there, and it’s both the bottleneck for concurrent users as well as the bulk of the page generation in typical setups. To make the biggest difference there you need good database design and data object caching with something like memcached, but here we’ll go over the options to improve your PHP execution times.
Alternative PHP Cache – http://pecl.php.net/package/APC
eAccelerator – http://eaccelerator.net/
XCache – http://xcache.lighttpd.net/
Zend Platform – http://www.zend.com/products/platform
ionCube PHP Accelerator – http://www.php-accelerator.co.uk/
Turck MMCache – http://turck-mmcache.sourceforge.net/index_old.html
On the able2know Q&A site we have used Zend and Turck MMCache in the past, with favorable results, but I am on a scalability and performance crusade here, and wanted to pick out the best of the current crop. Turck MMCache has not been actively developed for a while now, and is not a viable option for us to use in production, so that one’s out. I did the research into a variety of benchmarks (see chart), and with few exceptions the main competitors perform close enough to each other to make the performance differences less of a deal-killer in a selection between them. Simply put, the marginal performance gains you may acheive by selecting one over the other may be outweighted by differences in things like price, or how actively the code is being developed.
So I narrowed the selection down to XCache, APC and Zend. Zend does much more than PHP caching, and may be the right choice for others but they are a proprietary option that you must pay for, and that doesn’t justify the cost difference through performance gain as a PHP accelerator. XCache and APC are developed by well known programmers in the open source world, being the developers of lighttpd and PHP itself respectively. But between the two I am opting to use APC on able2know, as it is a pecl extension maintained by the maintainers of PHP itself (including the creator of PHP) and is reportedly going to become a core part of PHP 6.
So at least for me:
The best PHP accelerator to use is APC.