Using CI/CD on GitLab with CI4

Hello, everyone.
At my work, we created a framework based on CI2, later upgraded to CI3, but not to CI4 due to retro compatibility issues. We have over 30 applications using this framework, with our homemade deploy system and not great integrations with modern standards.
To improve our workflow, we changed our git server to GitLab, so we could have a ton of new features, such as CI/CD. Now that everything is set up, I'm not sure how I could begin to put this to work. First I thought that, as CI3 is relatively old, it would not be easy to integrate with this kind of thing. But after searching for guides for CI4, I cannot find eighter. Since CI4 came around, we discuss upgrading our framework to it, and this could be a good reason to do it, but if we won't be able to use CI/CD with CI4, I don't see the point to spend the effort.
My question is: is there a guide, or does anyone have the knowledge, on how to use CI/CD in GitLab on a CI4 project, or even CI3?

I’m using a hybrid GitLab/GitHub solution at my day job, so I don’t have a full GitLab CI/CD to share but here’s a starting point at least: https://github.com/duke-bluesmith/bluesm...lab-ci.yml

I have not a public project using CodeIgniter on GitLab CI/CD.

But this file can help on your custom configuration: https://gitlab.com/aplus-framework/proje...lab-ci.yml

The keywords reference is here: https://docs.gitlab.com/ee/ci/yaml/

There are very small changes to make it work with CodeIgniter 4.

 natanfelles.github.io

Thank you its huge help

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