Resource values like those returned by ImageCreate() are integers, despite when
they are echo'ed they expand to something like "Resource id #nnn".

However, it seems that when a resource value is used and array index,
Unset() does not take it as an integer unless you explicitly cast the
value.  Try this script to see the problem.

<?
 $resource=ImageCreate(1,1);
 $array=array();
 $array[$resource]=1;
 Unset($array[$resource]);
 echo IsSet($array[$resource])," ",serialize($array)," ",$resource," ",intval($resource),"\n";
 Unset($array[intval($resource)]);
 echo IsSet($array[$resource])," ",serialize($array)," ",$resource," ",intval($resource),"\n";
?>