More and more php compilers are emerging, and it now finally looks like there are a number of decent ones out there.
Just a couple of years ago, there only were bcompiler, eaccelerator and perhaps some others, but none really good. Eaccelerator would be sensitive to server environment and only work under certain conditions, and even have special problems with different php programs, and others had similair problems.
But now it seems like there are 5-10 different compilers, and all seem to work really nice. An especially interesting one is HipHop for PHP – a php compiler written by facebook to run in their own servers. The ultimate goal of all compilers is the SPEED (and, hwell, perhaps, obsfucation too), and I can only imagine what kind of speed problems the facebook experiences, with all their traffic, so if hiphop is good for them, then it should be too for the rest of the normal mortal aplications out there.
One important thing to know though is that hiphop for PHP is not an opcode cacher or a JIT. It is actually a compiler, and produces binary code as such. Probably because of that it has problems with eval() and some other dynamic features of PHP. If that is the case with your application, then it’s probably not for you, but how often does one really use eval?
Definetely check it out if you are looking to speed up your php application without changing a line of code. http://developers.facebook.com/blog/post/358/