Untitled

<head>
  <meta name="viewport" content="width=device-width, initial-scale=1" />
  <script type="text/javascript" src="js/jquery-2.2.4.min.js"></script>
  <script type="text/javascript" src="js/jquery.mobile-1.4.5.min.js"></script>
  <link rel="stylesheet" href="css/jquery.mobile-1.4.5.min.css" />
  <link rel="stylesheet" href="css/style.css" />

  <script type="text/javascript">
    $(document).ready(function() {
      $("#insert").click(function() {
        var username = $("#username").val();
        var password = $("#password").val();
        var dataString = "username=" + username + "&password=" + password;
        if ($.trim(username).length > 0 && $.trim(password).length > 0) {
          $.ajax({
            type: "POST",
            url: "http://localhost/jan/login.php",
            data: dataString,
            crossDomain: true,
            cache: false,
            success: function(data) {
              if (data == "success") {
                window.sessionStorage.setItem("loggedIn", 1);
                window.sessionStorage.setItem("username", username);
                window.location.replace("index.html");
              } else if (data == "error") {
                alert('error');
              }
            }
          });
        }
        return false;
      });
    });
  </script>
</head>

<body>
  <div data-role="page">
    <div data-role="header"><h1>Login</h1></div>
    <div data-role="content">
      <div class="list">
        <div class="item">
          <label>Username</label>
          <input type="text" id="username" value="" />
        </div>
        <div class="item">
          <label>Password</label>
          <input type="password" id="password" value="" />
        </div>
        <div class="item">
          <input type="button" id="insert" class="button button-block" value="Login" />
        </div>
      </div>
    </div>
    <div data-role="footer" class="center">
      <a href="register.html">Register</a>
    </div>
  </div>
</body>

</html>

<?php
    include 'db.php';

    $data=array();
    $q=mysqli_query($con,"select password from users where username="{$_POST['username']}"");

    if(mysqli_num_rows($q) === 0)
        die('error');

    $row=mysqli_fetch_assoc($q);

    if($row['password'] == $_POST['password'])
        echo 'success';
?>