Urgent Checkout Bug Fix

My e-commerce site runs smoothly until customers reach the checkout. Right now two issues are hurting conversions:

• Payment processing errors occasionally stop transactions even when the gateway is live
• Form-level validation sometimes lets empty or malformed fields slip through, which then triggers server-side rejects

I need an experienced developer who can dive straight into the existing PHP / JavaScript stack, trace the REST-API calls, and clean up the XML / JSON payloads involved. You will:

• Isolate and patch the faulty checkout code, ensuring every card or PayPal attempt (via our standard gateway SDK) completes without error
• Tighten client- and server-side validation so bad input is caught instantly, with clear inline messages
• Clear all console warnings and network errors related to the above
• Regression-test the fixes on staging, then push to production once we both confirm a clean run

Please be comfortable with PHP, JavaScript, HTML, CSS, MySQL, and inspecting API traces in real time. I expect the job to take no more than three hours; a concise change log and proof-of-success screen recording will complete the hand-off.