you setting player score to blank:
$playerScore = '';
then settng old score to player score, which is blank
$oldScore = $playerScore;
thereby over writing it....
try storing player score as a session:
if(!isset($_SESSION['score'])) $_SESSION['score'] = 0;
$_SESSION['score'] = $_SESSION['score'] +$_POST['playerScore'];
something along those lines.
this is not really a forum for general support on coding PHP. it is for support of webassist extensions, you may find a more helpful answer with full code examples by posting to the stackoverflow forums instead.