Give them a bug, in your real software product, that traces back to an operating system level setting, and does not initially expose this in the error. (for instance, say max. open files is set to 20 on the box, and a php script opens 100 file handles and doesn't close them)
Tell them to trace this, and suggest a fix, and give them a couple of hours.
If you can debug an environment you don't know, it proves that you're able to understand new concepts, and even trace weird bugs in them. Any monkey can program PHP, anyone with enough time can get a degree, not much guys know how to find a bug properly and fast.