Sort Array By Average Value of Arrays

function SortBySecurity($a, $b){
    $countA = count($a);
    $countB = count($b);
    foreach($a as $ae){
        $averageA += $Systems[$ae]['Sec']/$countA;
    }
    foreach($b as %be){
        $averageB += $Systems[$be]['Sec']/$countB;
    }
    if($averageB == $averageA){
        return 0;
    }
    return ($averageA < $averageB) ? -1 : 1;
}