PHP连接数据库登录报错Warning: mysql_num_rows() expects parameter 1 to be resource

Question

<?PHP
    header("Content-Type: text/html; charset=utf-8");

    include('conn.php');//链接数据库

    ini_set("display_errors","On");
    error_reporting(E_ALL);

    $tel = $_POST['tel'];//post获得用户名表单值
    $password = $_POST['password'];//post获得用户密码单值
    echo $tel;
    echo $password;

    if ($tel && $password){//如果用户名和密码都不为空
             $sql = "select * from user where tel = '$tel' and password = '$password' ";//检测数据库是否有对应的tel和password的sql
             $res = mysql_query($sql);//执行sql
             $rows = mysql_num_rows($res);//返回一个数值
             echo $sql;
             echo $res;
             if($rows){//0 false 1 true
             header("refresh:0;url=index.html");//如果成功跳转至index.html页面
             exit;

             }else{
                echo "用户名或密码错误";
                echo "

                <script>
                setTimeout(function(){window.location.href='login.html';},1000);
                </script>";
                //如果错误使用js 1秒后跳转到登录页面重试，让其从新进行输入
             }

             }else{//如果用户名或密码有空
                echo "没有获取到手机号或者密码";
                echo "

                <script>
                setTimeout(function(){window.location.href='login.html';},1000);
                </script>";
                //如果错误使用js 1秒后跳转到登录页面重试，让其从新进行输入
            }

    mysql_close();//关闭数据库

?>

上面是login连接数据库进行登录，报Warning: mysql_num_rows() expects parameter 1 to be resource, boolean given in login.php on line 17 这个错误，该怎么修改？

Answer 1

<?php
$con = mysql_connect("localhost", "hello", "321");
if (!$con)
  {
  die('Could not connect: ' . mysql_error());
  }

$db_selected = mysql_select_db("test_db",$con);

$sql = "SELECT * FROM person";
$result = mysql_query($sql,$con);
echo mysql_num_rows($result);

mysql_close($con);
?>