PHP二分查找的疑惑

Question

$Arr = array(1,3,4,5,7,8,9);
Search($Arr,7,0,count($Arr)-1);function Search($Arr,$FindVal,$LeftIndex,$RightIndex){
  echo "$FindVal,$LeftIndex,$RightIndex\n"; //加上这句
  if($FindVal>$Arr[count($Arr)-1]){
    echo "找不到该值";
  }else if($FindVal<$Arr[0]){
    echo "找不到该值";
  }else{
    $MiddleIndex=round(($LeftIndex+$RightIndex)/2);
    if ($Arr[$MiddleIndex]<$FindVal){
      Search($Arr,$FindVal,++$MiddleIndex,$RightIndex);
    }else if($Arr[$MiddleIndex]>$FindVal){
      Search($Arr,$FindVal,$LeftIndex,--$MiddleIndex);
    }else{
      echo "找到下标为$MiddleIndex";
    }
  }
}7,0,6
7,4,6
7,4,4
找到下标为4