Er det et sikkert log ind system?

PHP kode 

 $stmt = $mysqli->prepare("SELECT * FROM `users` WHERE username='$username' AND password='$password' AND SPS='$SPS'");
                             $stmt->bind_param('t', $_GET['username']);
                             $stmt->execute();
                             $result = $stmt->get_result();
                             $row = $result->fetch_assoc();    
                             if ($row = 1) {                            }

PHP kode 

 if (checkuserexists = 1) {
 echo 'User exists';
 } else {
 echo 'No user found.';
 }

PHP kode 

 <?php
                                 
                 if ($sid && $sbrugernavn) {
                     echo 'Welcome ' . $sbrugernavn;    
                 } else {
                     if ($_POST) {
                         $username = htmlentities($_POST['username']);
                         $password = $_POST['Password'];
                         $SPS = $_POST['SPS'];
                         
                         if ($username && $password) {
                             $password = md5($password  . $username . $salt);
                             $password = sha1($password);
                             $SPS = md5('s334sytgehnfyt3hrfen4' . $salt . 'feshnDE45r' . $password . $SPS);
                             $SPS = sha1($SPS);
                             $stmt = $mysqli->prepare("SELECT * FROM `users` WHERE username='$username' AND password='$password' AND SPS='$SPS'");
                             $stmt->bind_param('t', $_GET['username']);
                             $stmt->execute();
                             $result = $stmt->get_result();
                             $row = $result->fetch_assoc();    
                             if ($row = 1) {
                                 
                                 $user_info = $mysqli->query("SELECT * FROM `users` WHERE username='$username'");
                                 while ($row = $user_info->fetch_assoc()) {
                                     echo $row['id'];
                                     $_SESSION['id'] = $row['id'];
                                     $_SESSION['username'] = $username;
                                     
                                     header('Location: index.php');
                                 }
                             } else {
                                 echo 'Wrong username or password.';    
                             }
                         } else {
                             echo 'All fields is requried!.';    
                         }
                     }
                 ?>
              <form action="index.php" method="post">
                 <label for="username">Username *</label> <br />
                 <input type="text" name="username" id="username" placeholder="Write your username"/> <br />
                 
                 <label for="Password">Password *</label><br />
                 <input type="password" name="Password" id="Password" placeholder="Write your Password"/><br />
                 
                 <label for="SPS">Your speical password (can be anything)*</label><br />
                 <input type="password" name="SPS" id="SPS" placeholder="Write your Speical Password"/><br /> <br />
                 
                 <input type="submit" value="Login" />
                 
             </form>
             <?php
             }
             ?>