1: [Diagnostic] Work out under what circumstances this goes wrong
2: [Bug fixing] Fix at least one of the things you found in the above
3: [Simple extra functionality] Show the product of the three numbers as well as the sum
4: [More complex functionality] Show the biggest of the three numbers
5: [Refactorisation within PHP] Move the calculations into a class
6: [Wider Refactorisation] Dynamically add additional inputs (starting with just one). The calculations should still be done server-side.
DON'T PANIC: No-one has ever completed all of these in the interview :-)