It is obviously necessary to log the keys used as so we don't overwrite previous entries in the array ;o)

N.B. this also effects the timings, but still faster than the alternatives : 

     * int : 0.00159    (ms)
     * str : 0.00092    (ms)

<?php
private function _keySort(Array $data) 
    {
        $keys = array();
        
        foreach($data as $row) {
            
            $keyIncrement = 
                (!isset($keys[$row[$this->_orderField]])) 
                    ? $keys[$row[$this->_orderField]] = 0
                    : ++$keys[$row[$this->_orderField]];

            $tempArray[$row[$this->_orderField].$keyIncrement] = $row;
        }

        if ($this->_orderDirection == 'DESC') {
            krsort($tempArray, SORT_NATURAL | SORT_FLAG_CASE );
        } else {
            ksort($tempArray, SORT_NATURAL | SORT_FLAG_CASE);
        }

        return $tempArray;           
            
    }
?>