Be careful of the type of the parameter you pass to this, as it can have strange results...
<?php
// integers
ctype_alnum(1); // false
ctype_alnum(0); // false
ctype_alnum(-1); // true
// strings
ctype_alnum("1"); // true
ctype_alnum("0"); // true
ctype_alnum("-1"); // false
?>
Using PHP 4.3.11 on Windows/Apache