php二分查找

<?php

function binarySearch($arr,$findMe)
{
    if(0 == ($cnt = count($arr))){
        return -1;
    }
    $startIndex = 0;
    $endIndex = $cnt - 1;

    while($startIndex <= $endIndex){
        /**
         * 直观的写法 
         * $mid = floor($startIndex + ($endIndex - $startIndex) / 2);
         */
        /**
         * 下面的写法我一下子还不明白咋回事。。
         * 证明：
         * 设：起始位置x，结束位置为y。则中间：
         * x + (y - x)/2 = mid;
         * 2x + (y - x) = 2mid;
         * x + y = 2mid;
         * (x + y) / 2 = mid;
         */ 
        $mid = floor(($endIndex + $startIndex) / 2);
        if($arr[$mid] == $findMe){
            return $mid;
        } 
        if($arr[$mid] > $findMe ){
            $endIndex = $mid - 1;
        }elseif($arr[$mid] < $findMe) {
            $startIndex = $mid + 1;
        }
    }
    return -1;
}
$datas = array(1,2,3,4,5,6,7,8,9,10,11);
$foundedIndex = binarySearch($datas,'7');
var_dump($foundedIndex);