It's generally safe for a defender to assume that if an attacker can corrupt the heap they can achieve RCE, because there are usually so many ways to do it. They're all very specific ways that are hard to find because of all the complexity and mitigations, but there are a practically unlimited supply if someone puts in the effort to find one. But as a defender you don't need to, you should just say game over as soon as you get heap corruption, and go fix it.

Generally the attacker has to cause a sequence of heap allocations so that a certain kind of object gets allocated into the freed space that's still being pointed to, then use the old pointer to corrupt that object, then use the object. A good target is any C++ object with a vtable pointer - not sure if PHP is written in C++. If you can rewrite a vtable pointer and make it point to attacker-controlled data, you can get execution to jump to an attacker-controlled address.