How safe is PHP?

After having these crashes which I think were due to file upload (but who knows!) recently, I've been mulling the question: how safe is PHP? Namely how effective is the memory_limit directive? Forgive me because I don't know enough about PHP's architecture but is this memory_limit directive an absolute? Is there absolutely positively no way a child process can use more than memory_limit? Or are there situations where it can be exceeded, e.g. can certain bugs in PHP modules cause it to be ignored? An additional question is does Apache's RLimitMem/CPU directives work on PHP? Or is this only applicable to CGI processes? Any response appreciated. Nick.