My whole PHP file

<?php
        function Login() {
            $success = false;
            $usr = "XXX";
            $psw = "XXX";
            alert("HELLOOOOO");
            try {
                $con = new PDO( 'mysql:host=localhost;dbname=exalumnes', $usr, $psw );
                $sql = "SELECT * FROM Usuaris WHERE NomUsuari = :username AND Password = :password LIMIT 1";
                                      //users       //username

                alert($sql);

                $stmt = $con->prepare( $sql );

                $stmt->execute(array(':username'=>$_POST['username'], ':password'=>$_POST['password']));

                $valid = $stmt->fetchColumn();

                if( $valid ) {
                    $success = true;
                    session_start();
                    session_regenerate_id();
                    $_SESSION['user'] = $_POST['username'];
                    session_write_close();
                    echo ('CORRECTO');
                    exit();
                }

                $con = null;
                return $success;
            }
            catch (PDOException $e) {
                echo $e->getMessage();
                return $success;
            }
        }
?>