Function name must be string

Fatal error: Uncaught Error: Function name must be a string in
C:xampphtdocschatboxindex.php:5 Stack trace: #0 {main} thrown in
C:xampphtdocschatboxindex.php on line 5

    <?
    if(isset($_GET()["userId"]))
    {
        $_SESSION["userId"] = $_GET["userId"];
        header("location: chatbox.php");
    }
    ?>