what kind of a function can I use that will tell me whether a variable is
alphanumeric (that is has only characters A-Z or 1-9 )?
http://us3.php.net/manual/en/function.ctype-alnum.php
bool ctype_alnum ( string text )
Checks if all of the characters in the provided string, text, are alphanumeric. In the standard C locale letters are just [A-Za-z] and the function is equivalent to preg_match('/^[a-z0-9]*$/i', $text).
Returns TRUE if every character in text is either a letter or a digit, FALSE otherwise.
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Note that if you pass in an empty string you'll also get TRUE...
-philip