PHP form - WHERE problem ----Please HELP----

[Kostas Tsirigos]

Hello everyone!
I have created page with a form where the user can supply data in several fields.
The question is, when PHP gets all data supplied by the user (by using $_POST variable) and a general SQL query is ready, in the WHERE clause of the query, how will I insert the values submitted by the user?
Let's say you have 3 fields in the form: name, age, color
the SQL query is : SELECT ...... FROM ...
WHERE ???

Which value will be first inserted? I am asking this, because the user may not supply info about the field 'name' and may only be interested in searching about 'age'.