Are you wondering:
What is Opcode Cache and When to Use it?
I just read an excellent article entitled PHP Performance I: Everything You Need To Know About OpCode Caches published by Engine Yard. The article is a transcript of a video on PHP opcode cache, actually. Engine Yard goes into just enough detail, allowing one to really appreciate how it works.
What is OPCODE CACHE
OpCode Caches are an extension for PHP to enhance performance. By design, they inject themselves into the execution life-cycle and cache the results. This allows the compilation step to be skipped on subsequent cycles.
When to use OPCODE CACHE
According to the article, Opcode cache has almost no side-effects aside from using some extra memory for storing the cache. That stated, opcode cache should always be used in production environments
I recommend reading the article for a more indepth explanation. It’s a good read.