警告：mysqli_num_rows（）期望参数1为mysqli_result，布尔值为

Question

大家好我是PHP的新手，想要创建登录/注册。我想通过电子邮件检查用户是否已经存在。

但是我收到了这个错误：

 Warning: mysqli_num_rows() expects parameter 1 to be mysqli_result, boolean given

这是我的PHP代码：

<?php   

require "init.php";
$vorname = $_POST ["vorname"];
$nachname = $_POST ["nachname"];
$geburtstag = $_POST ["geburtstag"];
$geschlecht = $_POST ["geschlecht"];
$email = $_POST ["email"];
$namedeinerschule = $_POST ["namedeinerschule"];
$benutzername = $_POST ["benutzername"];
$passwort = $_POST ["passwort"];

$sql = "select * from user_info where email like '".$email."';"; 

$result = mysqli_query($con,$sql);
$response = array();

if (mysqli_num_rows($result)>0) 
    {       
        $code = "reg_faild";
        $message = "User already exist";
        array_push($response,array("code"=>$code,"message"=>$message));
        echo json_encode($response);
    }   
else
    {   
        $sql = "insert into user_info values ('".$vorname."','".$nachname."','".$geburtstag."','".$geschlecht."','".$email."','".$namedeinerschule."','".$benutzername."','".$passwort."');"; //speicherung
        $result = mysqli_query($con,$sql);
        $code = "reg_success";
        $message = "Thank your for register with us. Now you can login.";
        array_push($response,array("code"=>$code, "message"=>$message));
        echo json_encode($response);        
    }   
mysqli_close($con);
?
>

有人能帮助我吗？