PHP出现mysqli_num_rows() 期望参数 1 为 mysqli_result，布尔值在第25行的D:\genshi\postReg.php中给出的错误，该如何修改

问题相关代码，请勿粘贴截图
<?php
//trim去掉中间空格
$u_name = trim($_POST["u_name"]);
$u_password = trim($_POST["u_password"]) ;
$r_password = trim($_POST["r_password"]) ;
//第一步，连接数据库服务器
$conn = mysqli_connect("localhost","root","root",);
if (!$conn){
die("连接数据库失败");
}
//第二步，设置字符集
mysqli_query($conn,"set name utf-8");
//进行验证
if (!strlen($u_name) || !strlen($u_password)){
echo "";
exit;
}
if ($u_password <> $r_password){
echo "";
exit;
}
//判断用户名是否被占用
$sql = "select * from users where u_name = '$u_name'";
$result = mysqli_query($conn,$sql);//返回一个记录集
$num = mysqli_num_rows($result);//显示行记录数
if ($num){
echo "";
exit;
}
//sql语句
$sql = "insert into genshi (u_name,u_password) values ('$u_name','".md5($u_password)."')";
$result = mysqli_query($conn,$sql);
if ($result){
echo "";
}
else{
echo "";
}
运行结果及报错内容

mysqli_num_rows() 期望参数 1 为 mysqli_result，布尔值在第25行的D:\genshi\postReg.php中给出

$sql = "insert into genshi (u_name,u_password) values ('".$u_name."','".md5($u_password)."')";

修改sql