question

I have a php question. I would like to enter a "while" loop, but: I want it to be like this: when $a has a value, while $a... when $b has a value, while $b... (same statements as with $a) when $a has a value, and $b has a value, while $a and $b this is based on a search system in which the user can enter a, b or a and b. Can you help me with my problem, or help me find someone who can?! Thanks. Dore van Hoorn.