Quick Sort Multidimensional Array

function quicksort($seq) {
    if(!count($seq)) return $seq;
    $pivot= $seq[0];
    $low = $high = array();
    $length = count($seq);
    for($i=1; $i < $length; $i++) {
        // The index at which you would like to sort
        $sortIndex = 0;
        if($seq[$i][$sortIndex] <= $pivot[0]) {
            $low [] = $seq[$i];
        } else {
            $high[] = $seq[$i];
        }
    }
    return array_merge(quicksort($low), array($pivot), quicksort($high));
}