<?php			

	function validate_alphanum($string)
	{
		$count = 0;
		preg_replace("/[^a-zA-Z0-9]/", "", $string, -1, $count);

		if($count != 0)
		{
			return false;
		}
	
		return true;
	}
	
?>

<?php			

	function validate_alphanum($string)
	{
		$count = 0;
		preg_replace("/[^a-zA-Z0-9]/", "", $string, -1, $count);

		if($count != 0)
		{
			return false;
			die();
		}
	
		return true;
	}
	
?>

http://robert.im

This function uses the preg_replace counter to check for any non-alphanumeric characters.

You can use this for username / password validation if you don't want any special characters to be used.

Validate an alphanumeric string

php

PHP