Player score update with PHP MySQL

I have a users table where I want to update the scores each time a user finishes the game. Unityscript part is working fine but after I post the score to the database it appears doubled or tripled. I post the score as int and also the table column is of int format. My PHP looks like this:

$username = ($_POST['username']);
$score = ($_POST['score']);

try {

	$db = new PDO("mysql:host=$host; dbname=$dbname", $db_user, $db_pass);
	$statement = $db -> prepare ("UPDATE users SET score = score + $score 
                                  WHERE username = :username"); 
	$statement->execute(array(':username' => $username));
catch(PDOException $e) {
    echo $e->getMessage();

Any help or advice is appreciated.

Because you are adding the current score to the previous score in your query by using

SET score = score + $score

which will add the current score to the previous score that was already present in the database.