Description:
------------
The function returns false if the string is '0'.

Reproduce code:
---------------
---
From manual page: function.ctype-digit#Notes
---

$tripSpace = '0';
		if(empty($tripSpace)||!ctype_digit($tripSpace)){
				$error = true;
				echo "<p>TripSpace is $tripSpace</p>";
				$posSpaceClass = "inValidField";
			}