Presumably I'd need to interact with a wallet.dat file. The basic flow of events would be as such:

User pays bitcoins to the server. Server can see how many bitcoins user paid. Server pays another user or the original user an amount of bitcoins.

The server needs to be able to do those three actions solely from php.

EDIT: I've read the "lazy api" stuff on the wiki, none of that is desired or possible because they depend on trusting outside services to be reliable.