Get Rid of Those Pesky Symbols

Okay... from my previous post, many of you helped me sort out my function problems and things seem to be working great.  However, one more thing I need to do is to be able to detect the presence of symbols (anything not a number or a letter) and display a message.  However, after checking through www.php.net, I was unable to find anything.  Is this done with a simple function or does it take something a little more involved?