Random questions assigned to each PHP user. Working with sessions

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HTML

PHP

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This is what index.php looks like, where the user registers:

<title>php
   

     <h3>Registration</h3>

     Login
     
     Password
     
     Repeat password
     
     Registration
     <a href="login.php">Already registered?</a>

 0) {
         $error = "A user with this login already exists!";
     }

     if (!$error) {
         session_unset();
         $_SESSION['username'] = $login;
         $query = "INSERT INTO `users` (`id`, `login`, `password`, `admin`, `points`) VALUES (NULL, '$login', '$password1', 0, 0);" ;
         mysqli_query($link, $query);
         echo "You have successfully created a user!";
         header("Location: test.php");
         exit();
     } else {
         echo $error;
     }
    
}
?>

test.php:

<title>Testing
   

Hello, $username<br>";
     } else {
         header("Location: index.php");
         exit();
     }
    
     $dateAndTime = date("d.m.Y H:i");
     if ($_SESSION[$username]['selected_questions'] === null) {
         $query = "SELECT * FROM questions ORDER BY RAND() LIMIT 5";
         $result = mysqli_query($link, $query);
         $elapsed_time = 0;
         $selected_questions = array();
         while ($row = mysqli_fetch_assoc($result)) {
             $selected_questions[] = $row;
         }
    
         $_SESSION[$username]['selected_questions'] = $selected_questions;
         $_SESSION[$username]['test_start_time'] = time();
         echo "<a>Start of test $dateAndTime, maximum test time 10 minutes</a>";
     }
?>


     $question<p></p>";
             echo "$answer1<br>";
             echo "$answer2<br>";
             echo "$answer3<br>";
         }
     }
     ?>
     Check answers
     ";
         $total_score = 0;
  
         $query = "SELECT * FROM questions";
         $result = mysqli_query($link, $query);
  
         while ($row = mysqli_fetch_assoc($result)) {
             $question_id = $row['id'];
             $correct_answer = $row['correct_answer'];
  
             if (isset($_POST["question_$question_id"])) {
                 $user_answer = $_POST["question_$question_id"];
  
                 if ($user_answer == $correct_answer) {
                     $total_score++;
                 }
             }
         }
         if ($elapsed_time > $max_allowed_time) {
             echo "The test time exceeded the allowed limit. The results were cancelled.<br>";
             $total_score = 0;
         }
         $update_query = "UPDATE users SET points = $total_score WHERE login = '$username'";
         mysqli_query($link, $update_query);
  
         echo "You scored $total_score out of a possible 5 points.";
  
         $_SESSION['points'] = $_SESSION['points'] + $total_score;
         $_SESSION[$username]['test_start_time'] = time();
         $elapsed_time = 0;
     }
     ?>

<a href="index.php">Return to home</a>

What I have tried:

Each user should be randomly selected 5 questions from the database. How do I record a user's questions in a session so that each user has their own questions and they don't change after the page is refreshed?