<?php
    function midSch($arr, $key, $up, $down){
        if($up <= $down){
            $mid = intval(($up+$down)/2);
            if($key == $arr[$mid]){
                return $mid;
            }elseif($key < $arr[$mid]){
                return midSch($arr, $key, $up, $mid-1);
            }else{
                return midSch($arr, $key, $mid+1, $down);
            }
        }
        return 'NOT FOUND';
    }
    
    $arr = array(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12);
    $rs = midSch($arr, 11 , 0 , count($arr)-1);
    print_r($rs);
?>