Prerequisite
Composer
This might seem trivial for some developers, yet composer must be available within your legacy application. Without it's autoloading capabilities, it can be difficult to integrate this package's components. Therefore, please ensure that composer is installed and available inside your legacy application.
PHP v7.4.x
Unfortunately this package is only available from Athenaeum v4.x, meaning that it requires PHP v7.4.x or higher. If your application only runs on older versions of PHP, then you should spend the time upgrading it. Your application stands to benefit greatly from this, both in terms of security, performance and new language features.
Please consult yourself with PHP Migration Guides, for additional information.
As the author of this package, I apologise if you somehow feel mislead. I have been working on this and many other packages by myself. There are limits as to how much backwards compatibility I can offer.
Other Requirements
See eventual remaining requirements on packagist or inside this package's composer.json.
Tools to help Upgrading
You are not the only developer, that has to deal with upgrading legacy applications. Many other developers have gone through the same. Some of these developers have written very useful tools that can help you a lot. Below is a small list of tools that will help you upgrade.
PHP Compatibility checker
The PHP Compatibility Checker for PHP_CodeSniffer is a really great tools for identifying outdated code, e.g. unsupported features. It can be a bit cumbersome to install and setup. Yet, once it is up and running, it will show you exactly what line(s) of code are not compatible with PHP 7.x, 8.x,...etc.
Symfony Polyfill
Sadly, it is not always easy to deal with outdated code. Sometimes you might hit a "brick wall" that requires lots of hours or perhaps days to get through. This is especially true, when your legacy application depends on PHP features that are no longer supported. Fortunately, Symfony Polyfill has a great library that provides backwards compatibility to several outdated features.
Rector
This is probably the coolest, and most powerful tool for upgrading outdated code. It's able to do so automatically! Based on a few rules, Rector will automatically refactor your code. According to their documentation, it can upgrade code from PHP v5.3.x, all the way to PHP v7.4.x. Check out their online demo to see it in action.
Other Tools?
There are many more tools available, than the ones mentioned above. Try searching packagist or your favorite search engine, whenever you feel stuck with some outdated lines of code.
Note
If you happen to know of other great tools, that can help developers upgrade their legacy applications, then please feel free to contribute to this page's content.
Tip for installing Tools
Depending on the tool(s) that you choose to use, they can sometimes cause conflicts with the dependencies that your legacy application might contain. To avoid having to deal with this, you can make use of Composer Bin Plugin. It allows you to install your binary dependencies in a different folder, whilst linking the package's binaries into your regular vendor directory.
Testing...?
Depending on your end-goals, perhaps it would be better if you start adding a few automated high-level tests for your legacy application. Afterwards, then you can start to refactor, redesign or add several new features. Nevertheless, bringing automated tests into your legacy application, is beyond the scope of this package. You may find some inspiration by reviewing the following testing frameworks Codeception, PHP Spec, Behat, PHPUnit... etc.