PHP Math test (With Sessions :O)

<?php

session_start();

if(!isset($_SESSION['highest']))  // If Session highest isn't set....
{
    $_SESSION['highest'] = 100; // Default is 100 as highest number
}

if($_POST['highest']) // If someone posts a new highest number....
{
    $_SESSION['highest'] = $_POST['highest']; // set it !
    $f2_msg = "<span style=\"background-color: green\">Highest Number Set !</span>";
}

if($_POST['next'])
{
    header("Location: age.php");
    exit;
}
elseif(!isset($_POST['answer'])) // If answer isn't posted ....
{
    $_SESSION['var1'] = rand(0, $_SESSION['highest']); // Then set the vars, based on highest number, hence why it is above
    $_SESSION['var2'] = rand(0, $_SESSION['highest']);
}
elseif(isset($_POST['answer']))
{
    $true_answer = $_SESSION['var1'] - $_SESSION['var2']; // Get the true answer
    $user_answer = $_POST['answer']; // Get the user's answer

    if($true_answer == $user_answer) { $f1_msg = "<span style=\"background-color: green\">Correct Answer !</span>"; } // Compare answers
    else { $f1_msg = "<span style=\"background-color: red\">Wrong Answer !</span>"; }
}

?>
<h3>Question :</h3><br/>
<b>What is <?php echo $_SESSION['var1'] ?> - <?php echo $_SESSION['var2'] ?> ?</b>
<br/><br/>
<h3>Answer :</h3><br/>
<?php if(isset($f1_msg)) { echo "$f1_msg<br/><br/>"; } ?>
<form method="post">
  Your Answer : <input type="text" name="answer">
  <input type="submit" value="Verify >"><tr/><input type="submit" value="Next Question">
</form>
<br/><br/>
<h3>Configuration :</h3><br/>
<?php if(isset($f2_msg)) { echo "$f2_msg<br/><br/>"; } ?>
<form method="post">
  Highest Number : <input name="highest" type="text" value="100">
  <input type="submit" value="Set >">
</form>