count($Array["nonexistent"]) sets $Array["nonexistent"]=""
Here is the script:
$T=array();
echo " isset: ".isset($T["a"])." count: ".count($T["a"]);
echo " isset: ".isset($T["a"])." count: ".count($T["a"]);

The result is: isset: 0 count: 1 isset: 1 count: 1, while I would expect
it to be: 0 0 0 0 (or 0 1 0 1).