USUALLY it's better not to do that. It eliminates the possibility of making some really interesting queries that can easily be made with your current design.
Honest, the PHP code to do this is trivial. I don't even use PHP, but I could write it (probably with some PHP syntax errors, but the logic of it is trivial).
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.