Good work of the functionality of selling entrance tickets on the site with a self-written PHP engine

Make an extension of the functionality of selling entrance tickets on 2 sites identical in technical terms. Now both sites are already working and they are selling.
Detailed description of the tasks in the attached file with the terms of reference. For the development of the project there is pre-production, all changes will be made on it and only after coordination and verification will be implemented for production.

Technical information:
- Self-written PHP engine. PHP version 7.1.
- Almost all the functions of the site are described in PHP and JS, but some functionality is moved to the database procedures.
- For each site there are different branches on bitbucket and you can work with the project through this version control system.

Based on the results of the work, you need a list of files in which changes were made or commits were made to bitbucket, as well as a list of changes to the database, if any.

All questions about the project (what and where lies, in what files, how it works, etc.) can be discussed in detail on Zoom.
The selected executor will be granted all the necessary access from pre-production: admin panel, FTP, database, delegation of access to the repository to bitbucket.org.

Only answers from freelance programmers are accepted for consideration. Agencies and intermediaries are not accepted for consideration.
Also important is the experience of working on pure PHP without using frameworks and engines. If there are any projects, add them when you reply to the project.