如何检查数据库中是否存在用户（已经以登录表单注册）？

Question

I've been trying to create a login page where users who sign up are written in a database. I want to check, when a user tries to log in, the code should find if the user is registered before(if exists in the table in the database). And if exists he should sign in and to be redirected to another page(in this case kontakt.php). Here's what I've done till now, but it shows me error: Warning: mysql_num_rows() expects parameter 1 to be resource, boolean given in C:\\xampp\\htdocs\\BookShop\\login.php on line 14 Is something wrong with the query?

<?php
session_start();
require "C:/xampp/htdocs/BookShop/DButils.php";

//opening a DB connection
$connection = @ mysql_connect($hostname, $username, $password);

//selecting DB
mysql_select_db($dbname, $connection);

$username=mysql_real_escape_string($_POST['username']);
$password=mysql_real_escape_string($_POST['password']);

$query = mysql_num_rows(mysql_query("SELECT * FROM kupuvac WHERE username = '$username'             AND password = '$password'"));

if($query == 1)
 {              

   header("Location:kontakt.php");

  }
else if ($query == 0 )
   {

    echo "Please try again";
   }
 ?>

Answer 1

mysql_query() returns a resource if the query is successful, but it returns FALSE if there is an error.

From PHP's website:

For SELECT, SHOW, DESCRIBE, EXPLAIN and other statements returning resultset, mysql_query() returns a resource on success, or FALSE on error.

Answer 2

You should always check your results for validity.

$result = mysql_query('SELECT * WHERE 1=1');
if (!$result) {
    die('Invalid query: ' . mysql_error());
}

$rows = mysql_num_rows($result);