You should probably have the score checking in a PHP script which your quiz form submits to. And in that score checking script, you should be checking $_GET['question1'] or $_POST['question1'], depending on your form submission method.
example form page:
<form action="grade.php" method="POST">
...
</form>
example score script:
<?php
$score = 0;
if ($_POST['question1'] == 'a')
++$score;
if ($_POST['question2'] == 'a')
++$score;
?>
YOUR SCORE IS: <? echo $score ?>