I recently released a proof-of-concept library for drawing force directed graphs with PHP. Right now, it is absurdly simple and has a few quirks, but it gets the job done. It's original purpose is for visualizing search results within a controlled vocabulary, although I can imagine other uses.

Right now, it will only properly function in browers that support the <canvas> tag, although that will change in a couple of weeks. I also hope to implement a few parsers to make it easier to create graphs (I'm thinking RDF and dot will be sufficient). There's also some necessary performance enhancements before it can be considered production-ready.

You can download the files here or see the library in context at the bvault repository on launchpad.

This work is inspired by the JSViz library which renders forced directed graphs on the client side using javascript.