Asterisk AGI Billing, PHP Asterisk VoIP Customer Portal

In the development of a PHP based VoIP user portal, we are seeking an Asterisk AGI script which will check a balance value in a MySQL database, compare to a rate table, determine how many seconds the call can last based on the preceding and allow the call to terminate appropriately. This script will then need to rate the call and deduct the appropriate amount from the balance and update the value in the mySQL table. The cdr table will also need to reflect the total amount billed for the call. In addition, we are seeking a PHP script for adding new peers to the server (asterisk-realtime). We will require the ability to have different billing methods, ie postpaid, certain amount of minutes included, then billed per minute.