I don't use PHP much, but I was curious, so here's the answer you don't want:

You get $a[1] = 0 the first way, and $a[0] = 0 the second way. PHP seems to evaluate the array index before the right hand side if there is something more than a $variable to evaluate; otherwise PHP evaluates the array index after the right hand side, after $i has been incremented.

I haven't used hiphop, but I'm curious if it preserves that disparity between arrays when translating to C++, or if it breaks PHP compatibility by making those two arrays equivalent.