Where can I find information about the architecture of popular engines written in PHP?

Question

CMS

I have been programming in php for more than 10 years, recently I have been working on one project, although initially I was hired as a backend developer, last year I did a complete redesign and improved my knowledge and skills in css and html; however, this affected the fact that little time was devoted to php-programming itself, and the engine is gradually becoming less and less convenient ... read more about architectures and design. So that I can get rid of this self-made engine and be able to participate in the development of projects that use a convenient architecture. For example, recently I was asked to get into osCommerce - and I was horrified at how terrible everything is displayed there through <?php echo '...';?>, it would seem that a popular system but has rather outdated giblets inside. The system I'm dealing with has:
In short, the question arose of finding a PHP CMS with an architecture similar to that used in my project and, possibly, has a number of advantages; and it would not hurt (announce the entire list)
-) a single php file that processes incoming requests (Limonade - PHP micro-framework),
-) modules that return XML are loaded from it;
-) XSLT files with the design are loaded (via XSLTProcessor::importStylesheet) and,
-) (via XSLTProcessor::transformToXML) data from the modules is "rendered" into the design from XSLT and displayed to the user.

Answer 1

Judging by the description, CMS is not particularly needed here, any microframework will do. Slim / Silex / Zen Expressive etc.
Architecture - see GitHub, everything is open.

Answer 2

And something newer and more complete?