I'm not seeing too much written in PHP. If you search a little deeper maybe you can come up with something at sourceforge.net. The wikipedia page you linked to gives some open-source ones that might interest you, however, they are not written in PHP. If you absolutely need it in PHP, and depending on what you need, I do not think it would be too hard to write one yourself. I guess you'll decide that for yourself, though.
Anyway, good luck on your quest! :)