Translation of a self-written PHP site from 5.6 to 7.2

It is desirable to work with a git repository. Step-by-step work and payment (then exactly with a git, commit at each stage) or work on a safe transaction. A small test task.

About the project: php directory site without OOP. Number of lines of code: 22934. Number of PHP files: 78. We will reset the archive of the site files with the git repository and the database dump.

What to do: replace mysql_* with mysqli_*, replace