Advanced PHP and Twig Code Optimization, Security Hardening, and Manual Refactoring

I have a web application built on custom PHP and Twig. Over time, this codebase has been modified 2 or 3 times by different developers. Because of these multiple past modifications, the internal codebase has become extremely messy and unstable... (Budget: ₹1000 - ₹2000 INR, Jobs: CSS, HTML, JavaScript, MySQL, PHP)