> Maybe there seem to be some alternatives?

This is not meant to be an endorsement of this method, but PHP <7.0 uses a single-pass compilation where the parser itself does the opcode compilation. A lot of the buggy and quirky behavior PHP exhibits (at least when that behavior is unintentional) is the result of this process.

Like a lot of extant PHP, there's no real reason it was done this way: it's just the method Rasmus et al happened to program the original engine, and it's grown organically since then. With the AST, phpng, and the uniform variable syntax, I'd argue PHP7's main feature is finally getting around to correcting the sins of the past.