//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;