variables in php are preceeded by a dollar sign.
$total = score + score1;
should be
$total = $_POST['score'] + $_POST['score2'];
I believe it is good practice to set the $_POST array into variables though, so it can be this as well:
$score1 = $_POST['score'];
$score2 = $_POST['score2'];
$total = $score1 + $score2;
$_POST[$total] should just be $total