HHVM is not a drop-in replacement for a PHP interpreter. The semantics of Hack and PHP have diverged, typically in the direction of eliminating dynamic behavior from Hack that existed in PHP (examples: string -> function coercion, the PHP dual vector-hash-table array type, and non-throwing out-of-bounds array accesses are all gone from Hack). The semantics changes both simplify static analysis and make it easier to JIT fast code.