Note that PHP 4 will generate a warning if you try to unset an array index that doesn't exist and whose parent doesn't exist.

Example:

<?php

  $foo = array();

  unset($foo['Bar']['Baz']);

?>

RESULT: "Notice:  Undefined index:  Bar"

On PHP5 no error is raised, which seems to me like the correct behaviour.

Note that using unset($foo['Bar']) in the above example does not generate a warning in either version.

(Tested on 4.4.9 and 5.2.4)