If you run the code on your own servers without distributing it, you don't have to release your source code. If you distribute your code which links to the GPL code, then you must release your source code. If you distribute any modified GPL code, you have to release the modifications, even if your code does not link to it.
Technically I think this means that if you use a hosting company, you are bound to distribute the source code to them when you install your code. Since PHP isn't compiled, this should just happen naturally.
What's linking? PHP (like Java) isn't statically linked, so the FSF defines linking as one piece of code directly accessing another on the same (virtual or physical) machine - like a procedure call or using a public variable. Merely distributing two unrelated pieces of code on the same CD or in the same zip file does not constitute linking. Merely starting a program is not linking.
IANAL and this is not legal advice, but rather my best guess.