Forms and PHP variables

Hi, First off, thanks to all those that helped out with my other questions. Not all my problems were solved, but I'm certainly closer. Still working on the same project I need to do the following. I have a form where users input numbers only, is there a way I can have php check to make sure a letter or other character didn't get put in by accident. I was looking at ereg, but not to familiar with this or if it would work in this case. TIA