How good are PHP accelerators? has an accelerator which almost matches Zend's own one, and it's free! So yes, it's worth it (and they are good).

In case you want to know how it works, this is how. When a file is called (index.php) the server reads that file, parses it, fills in all the gaps, loads all the other files, gets all the variables sorted, etc etc etc, and then runs the PHP executable to execute the page and produce the output. The problem is that this cycle is repeated every time the page is called.

What the accelerator does is presume that the page hasn't changed it's coding in between requests, therefore doesn't need reloading and recompiling. It stores the (almost) last product, and executes that! Simple!
And does it check to see if the code is newer than the compiled version?
(More than likely yes, but I just want to be sure)
Yes, not exactly sure of the mechanism but I think it checks the last changed date or something. It's got everything written up on the site, have a read!