函数名必须是json_encode中的字符串

I'm not sure what the problem is, I get the error "Function name must be a string":

<?php
    $prodId = $_GET('productId');
    echo json_encode(array( 'productId' =>  $prodId) );
?>

I just want to echo back a XMLHTTPRequest JSON request, to make sure the GET is working.

$prodId = $_GET('productId');

should be

$prodId = $_GET['productId'];