Ok, i have a form that is passed data through a QR CODE. The form works great at what it does. It asks for username and password and if they authenticate, then information is entered into MYSQL db. Works perfect. However, I am getting duplicate records. I don’t want duplicate records, but I’m also not a PHP programmer.. My question, can someone look at this code, and help me get to where it needs to be to:

1. Authenticate the user
2. Check if record already exists, and
3. If it doesn’t exist, enter into database ORT
4. if it does exists, die.
5. Show me how to update the code to work with PHP 8.1

 <?php

    require('db.php');
    
    Session_start();
    $FN = $_GET['FirstName'];
    $LN = $_GET['LastName'];
    $DL = $_GET['DLNumber'];



          if (isset($_POST['username'])) {
            $username = stripslashes($_REQUEST['username']);
            $username = mysqli_real_escape_string($con, $username);
            $password = stripslashes($_REQUEST['password']);
            $password = mysqli_real_escape_string($con, $password);
            
            $query    = "SELECT * FROM `onlineusers` WHERE username='$username'
                         AND password='" . md5($password) . "'";

            $result   = mysqli_query($con, $query)  or die(mysql_error());
            $rows = mysqli_num_rows($result);

            if ($rows == 1) {
                $_SESSION['username'] = $username;
                
                $Instructor = $_POST['username'];
                 $_SESSION['FirstName'] = $FN;
                             switch ($Instructor) {
                     case "Kevin":
                         $Instructor = 'Kevin D.';
                         break;
                     case "Carlos":
                         $Instructor = 'Carlos L.';
                         break;
                     case "Mike":
                         $Instructor = 'Mike T.';
                         break;
                     case "Bill":
                         $Instructor = 'Bill W.';
                         break;
                 }
                 
                
                 $Date = date('Y-m-d');
                 $Hours = '5';
                 $Activity = 'Training';
                 
                 $Location = $_POST['location'];
                 switch ($Location) {
                     case "Please select a location":
                         $Location = 'Chicago';
                         break;
                     case "Milwaukee":
                         $Location = 'Milwaukee';
                         break;
                     case "Mount Pleasant":
                         $Location = 'New York';
                         break;
                 }
$query = mysql_query("SELECT * FROM log WHERE DLNumber='$DL' AND Date='$Date'");

if(mysql_num_rows($query) != 0){
echo "Student already checked in for the day.";
}
else
{
$query2    = "INSERT into `log` (FirstName, LastName, DLNumber,Instructor,Location,Hours,Activity, Date)
                         VALUES ('$FN', '$LN', '$DL', '$Instructor','$Location','$Hours','$Activity','$Date')";

                $result   = mysqli_query($con, $query2);
                
                header("Location: dashboard.php");

}


          } else {
              echo "<div class='form'>
                    <h3>Incorrect Username/Password.</h3><br/>
                    <p class='link'>Click here to <a href='login.php'>Login</a> again.</p>
                    </div>";
          }
      } else {
?>
<form class="form" method="post" name="login">
       <h1 class="login-title">Login</h1>
       <input type="text" class="login-input" name="username" placeholder="Username" autofocus="true"/>
       <input type="password" class="login-input" name="password" placeholder="Password"/>
       <input type="hidden" class="login-input" name="FirstName" value='<?php echo trim($FN); ?>'/>
       <input type="hidden" class="login-input" name="LastName" value='<?php echo trim($LN); ?>'/>
       <input type="hidden" class="login-input" name="DLNumber" value='<?php echo trim($DL); ?>'/>
       <input type="hidden" name="login" value="true"> 
       <select name="location">
        <option hidden="true">Please select a location</option>
           <option value="Chicago">Chicago</option>
           <option value="Milwaukee">Milwaukee</option>
           <option value="New York">New York</option>
       </select>
       <input type="submit" value="Login" name="submit" class="login-button"/>
       
     </form>

<?php