//二分查找

$arr = array(0,1,2,3,4,5,6,7,8,9);

function bin_sch($array, $low, $high, $k){

    if ($low <= $high){

        $mid = intval(($low+$high)/2);

        if ($array[$mid] == $k){

            return $mid;

        }elseif ($k < $array[$mid]){

            return bin_sch($array, $low, $mid-1, $k);

        }else{

            return bin_sch($array, $mid+1, $high, $k);

        }

    }

    return -1;

}

$key = bin_sch($arr , 0 , count($arr) , 4);

echo $key;