PHP function to generate v4 UUID

So I've been doing some digging around and I've been trying to piece together a function that generates a valid v4 UUID in PHP. This is the closest I've been able to come. My knowledge in hex, decimal, binary, PHP's bitwise operators and the like is nearly nonexistent. This function generates a valid v4 UUID up until one area. A v4 UUID should be in the form of: xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxx