Array references - how to unset() ?

$ARR = array( 'a' => array('b' => 'blah') ); function set($key) { global $ARR; foreach ($key as $i => $k) {

        if ($i == 0) {
               $sourcevar =& $ARR[$k];
           } else {
               $sourcevar =& $sourcevar[$k];
           }
       }

    // unset($sourcevar); // will cancel the reference - we want to unset the ['b'], but how?

$sourcevar = null; // will set it NULL, but won't unset... foreach ($ARR ... // I could run a cleanup, that would go through all of the array and unset what is NULL, but I would need to use REFERENCES again!! array_walk_recursive() is also worthless... any ideas? } set( array('a', 'b') );