1) So what about the (I would think fairly common case) of loops where the variable is first initialized in the loop. On one path, it's null, on the other, it's a typed value. Can you optimize that?

2) Cool! phc went the route of really advanced, context sensitive analysis, and the result was massive memory use and huge compile times for even simple programs (if using optimization).

3) Have you read IBM's POPL 2010 paper on it? Interesting stuff, though maybe not actionable. They argue (there's that word again!) that PHP's copy-on-write is flawed for arrays containing references - the reference can lose it's reference-ness when the array gets altered and copied. They have an implementation which fixes this for a relatively small slowdown. Interesting paper as I recall.

FWIW, I think it's allowable to change the output of var_dump. Optimizing reference counts kinda requires it, and I know Hiphop isn't in the business of strict conformance to PHP semantics.