In this recent post to his blog Stuart Herbert introduces a system he's created to handle "contracts" in PHP development - ContractLib.

Programming contracts are tests around functions and methods, and they are normally used: to catch any 'bad' data that has been passed into the function or method from the caller, and to catch any 'bad' data generated by the function or method before it can be returned to the caller. These are pre-condition and post-condition tests, and they are tests that either pass or fail.

He points out that by having contracts you not only increase the robustness of your code but you also save time not trying to hunt down data-related issues. Using pre-conditions, you can can check data to ensure things like correct formatting, data that's out of range and data that might be missing. His ContractLib comes with a set of tests that provide good examples of how to use the functionality. Installation instructions are included.