Fix Payment Gateway Integration

I need an experienced CodeIgniter developer to fix a payment gateway integration issue on an existing Codecanyon script (Affiliate Management System). The payment gateway (PayFast) worked previously but now fails with an "Authentication error" after a script upgrade.

Requirements:
- Expertise in CodeIgniter 3
- Experience with PayFast integration
- Ability to troubleshoot and resolve authentication errors
- Work must be done in the production environment

Please only apply if you have relevant experience and can ensure a smooth, secure fix.