php 2分查找， return true结果var_dump()是null

function searchNum($arr,$num,$start=0,$end=null){
    if($start>$end){
        return false;
    }
    $count=count($arr);
    if(!$end){
        $end=$count-1;
    }
    $mid=floor(($start+$end)/2);
    echo $num,'<br>';
    echo $mid,' : ',$arr[$mid],' ';
    if($arr[$mid]==$num){
         echo true;
        return true;
    }
    elseif($arr[$mid]>$num){
        searchNum($arr,$num,$start,$mid-1);
    }
    else{
        searchNum($arr,$num,$mid+1,$end);
    }
}
$arr1=[1,2,3,15,18,21,23,34,38,42,45,48,68,77,79,81,84,88,91,99,102,105,110];
var_dump(searchNum($arr1,45));

<?php 


function searchNum($arr,$num,$start=0,$end=null){
    if($start>$end){
        return false;
    }
    $count=count($arr);
    if(!$end){
        $end=$count-1;
    }
    $mid=floor(($start+$end)/2);

    if($arr[$mid]==$num){

       return true;

    }
    elseif($arr[$mid]>$num){
        return searchNum($arr,$num,$start,$mid-1);
    }
    else{
       return searchNum($arr,$num,$mid+1,$end);
    }
}


$arr1=[1,2,3,15,18,21,23,34,38,42,45,48,68,77,79,81,84,88,91,99,102,105,110];

    $res=searchNum($arr1,45);
    var_dump($res);

你的在 

searchNum($arr,$num,$mid+1,$end);

这里加return 不然