JIT (just in time) compilation is, according to Wikipedia, a method for converting, at runtime, code from one format into another, for example bytecode into native machine code. To try to see the benefit of this, Jacob asks four questions in his post (each with their own problems):

• Q: What advantage over opcode caching is gained?
  P: A JIT library that works with PHP would have to support all of the architectures that PHP can run on!
• Q: Which JIT library is the best one?
  P: A JIT compiler is not going to achieve anything without either storing the results or caching them.
• Q: Wouldn't writing PHP extensions negate the need for JIT compilation?
• Q: When will I develop and release my implementation?