site.inc.php error

Hi, I am trying to create a function in the site.inc.php file that uses the $db->get_payments($payment_id); hook. But when I try to use this hook according to the amember manual, I get the error message: "Fatal error: Call to undefined function setup_plugin_hook() in /home/wikicook/public_html/amember/site.inc.php on line 21"

I am getting the error message when I type www.wikicook.com/amember/site.inc.php directly into the browser. I know the error means that the code doesn't interpret $db as an object. But I declared 'global $db;' like the amember manual told me to so I don't know why it isn't.

Any help would be appreciated!
Thanks!

I have copied the function below - this is the complete code of the site.inc.php file.

___________________
<?php
error_reporting(E_ALL);

function get_active_member_array(){
//this function assumes users can only subscribe to one product at a time.

global $db;

$active_member_array =
$db->query($sql= "SELECT member_id
FROM {$db->config[prefix]}members
WHERE status = '1'
ORDER BY member_id"); //This statement begins on line 21, which the error message refers to.

return $active_member_array;
}

//Testing the Function by printing the member array:
$member_array =
get_active_member_array();

echo "active member array:<br>";
echo "<pre>";
print_r($member_array);
echo "</pre>";

?>