Fix Coupon code issue in a PHP website

I have a PHP website with a coupon code issue that's affecting the checkout process. I'm looking for a skilled developer to resolve this problem.

Key tasks include:
- Investigating the existing coupon code functionality
- Identifying the cause of the issue
- Fixing the coupon code to ensure it behaves as expected
- Testing the checkout process thoroughly post-fix

Ideal candidates should have:
- Proficient experience with PHP, and particularly PHP checkout processes
- Strong problem-solving skills
- Good knowledge of eCommerce coupon code functionalities
- Excellent testing capabilities to ensure the solution is fully implemented and functional

Your ability to fix this promptly and accurately will be highly valued.