(I work on the HipHop compiler.) You start by compiling PHP source.

Simple PHP statements take a lot more space in the binary than intuition suggests. E.g.:

if ($a == $b) ...

would seem like it should be

  cmp $rax, $rbx
  jz ...

But! If type inference has failed, we don't know what types $a and $b are, so they might be strings or objects or something crazy. So we're going to have to indirectly dispatch to $a's '==' method. We also spend a ton of space on reference counting code; the semantics of the language basically force you to do naive reference counting, since refcounts can be witnessed in various ways, so every time we pass an argument, do an assignment, sometimes even evaluate expressions, we need to manipulate reference counts, and if they've gone to zero call a destructor.

It ends up making the code really large, and one of the things that's unique about our efforts to run PHP fast relative to other dynamic language efforts is that sheer code bulk ends up being our largest enemy; if we're not careful, icache misses eat us alive.

Finally, I'll note that it's not quite a 1.5GB binary. The actual ELF binary is something like 1.1GB, and the remainder of the package we bittorrent around production is stuff like static resources (javascript, css) and primed contents for the APC cache that we want prepopulated on boot.