I am using the following setup:
Zen Cart 1.3.9g
Server OS: Linux 2.6.32.8-grsec-2.1.14-modsign-xeon-64

So, I have built a custom page in the admin folder that allows admins to add points to a customer account. In this instance, points are like credits. The form page goes to a processing page and saves points to the database. This all works fine.

On the processing page, I am trying to use the zen_mail() function to notify the customer, but I get this error:
Email Error: Could not instantiate mail function. Please check Admin->Configuration->Email Options->Email Transport.

If I go to the Customers section, and send an email to a specific customer, it works fine, so I think the Email Transport settings are correct. It is currently set to PHP.

This is the code I am using:
$messageType = (substr($text_only_content, 0, 5) == '<?xml') ? 'xml_record' : '';
zen_mail($name, $address, $subject, $body, STORE_NAME, EMAIL_FROM, $html_msg, $messageType, '');

Is there something I need to do or some file I need to include in order to use the zen_mail() function?

Currently I have my php file including the following:
require('includes/application_top.php');
include_once('/includes/functions/functions_email.php');

Any help would be appreciated.