This is an exemple of all the worst PHP development practices you can follow when developing PHP in 2018. Its only purpose is to demonstrate what's wrong, and how it impacts on the whole software development, maintenance and release cycles, for training reasons.
You are recruited in a company to handle their legacy software.
You did ask for a full rewrite of the project, but the administration board voted against as they fear it will be too expensive given their past experiences.
Also, they do not want to enter a feature freeze period as the business actually rely on shipping new features.
The current software is a ticket booking system that allows the user to book one or many tickets to a single event.
It's written in PHP 5.3 and currently works on an old shared server, on a hosting company that is not reliable.
As the only developer on the project, you are free to use whatever tools suit your requirements, keeping in mind the code must be extensible and readable by any other developer.