Why does PHP use opcode caches while Java compiles to bytecode files?
From my point of view, both PHP and Java have a similar structure. At first you write some high-level code, which then must be translated in a simpler code format to be executed by a VM. One difference is, that PHP works directly from the source code files, while Java stores the bytecode in .class files, from where the VM can load them.
Nowadays the requirements for speedy PHP execution grow, which leads people to believe that it would be better to directly work with the opcodes and not go through the compiling step each time a user hits a file.
The solution seem to be a load of so called Accelerators, which basically store the compiled results in cache and then use the cached opcodes instead of compiling again.
Another approach, done by Facebook, is to completely compile the PHP code to a different language.
So my question is, why is nobody in the PHP world doing what Java does? Are there some dynamic elements that really need to be recompiled each time or something like that? Otherwise it would be really smarter to compile everything when the code goes into production and then just work with that.
Answer
The most important difference is that the JVM has an explicit specification that covers the bytecode completely. That makes bytecode files portable and useful for more than just execution by a specific JVM implementation.
PHP doesn't even have a language specification. PHP opcodes are an implementation detail of a specific PHP engine, so you can't really do anything interesting with them and there's little point in making them more visible.