Fix WooCommerce Payment Gateway Plugin Bug

Our e-commerce website was recently updated to WordPress 7+ and modern WooCommerce under a PHP 8.1+ architecture. Following this update, our legacy custom payment gateway plugin (v0.3.1) completely failed at checkout... (Budget: $10 - $30 USD, Jobs: API, Database Management, HTML, Payment Gateway Integration, PHP, Web Development, WooCommerce, WordPress)