This is a micropass compiler, translating the code in numerous simple steps. The more the merrier as that's easier to debug. The passes are split up into "ends": the front-, middle and back-end. All code has to go through the front-end before the other ends in order to collect as much data as possible required to generate code of acceptable efficiency.
Aside from gathering that information the front-end breaks down the code into assembly-style control flow made of goto statements only. It also moves expressions out of argument lists by assigning them to temporary variables which replace them in the argument lists. The middle-end connects the dots by building a tree of FUNINFO objects and performs tailcall and peephole optimizations. The back-end is left to generate the code. Stack places are assigned here if required (e.g. for C, bytecode or PHP closures).