Call to a member function prepare() on a non-object in

function connect() {
    try {
      
        $dbh = new PDO('mysql:host=' . HOST . ';dbname=' . DBNAME, USER, PASS);
        $dbh->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    } catch (Exception $e) {
        $error .= "Błąd nie można sie połączyc z bazą danych!: " . $e->getMessage() . "<br/>";
        $dbh = NULL;
    }
    return $dbh;

function update($id) {
    $dbh = connect();
        foreach ($id as $key => $value) {
            $stmt = $dbh->prepare("UPDATE `zapisy` SET `platnosc` =2 WHERE id =:id");
            $stmt->bindValue(':id', $value, PDO::PARAM_INT);
            $stmt->execute();
        }
}
}