PHP: Fatal error: Call to a member function insert() on a non-object

I get an error from using the function below: Call to a member function insert() on a non-object.
Please help!

<?php 

$hostname = "dd";
$username = "cc";
$password = "aa";
$dbName = "bb";
MYSQL_CONNECT($hostname, $username, $password) OR DIE("Unable to connect to database");
@mysql_select_db("$dbName") or die( "Unable to select database");

subscribe($email, $firstname, $lastname, $company);

function subscribe($email, $first, $last, $company)
{
             global $wpdb;  
             $wpdb -> insert('aca11052015_subscribers', array(
            'First Name' => $first,
            'Last Name' => $last,
            'Company' => $company,
            'Email' => $email,
            'Date' => date("Y-m-d H:i:s")
        ));    
}