SMS Integration for php text based game

We have an online mafia browsers text game whereby users can purchase credits through Paypal. This script is mccodes and is php. We would now like to offer users the ability to pay via mobile SMS. We are based in the UK and our users are worldwide so this would need to be considered. We would require someone who has an understanding of the cheaper SMS market providers and who would be able to integrate such into our game. Basically the existing checkout page which holds the Paypal payment buttons for credits would also hold the SMS payment button whereby in the same way the return from Paypal updates the customer’s account with the credits the same would occur with SMS payments. So the payments would be automated whereby SMS payment is made and the users account is updated. Thank you