I think the most important part is in https://github.com/phpbb/documentation/tree/3.1.x/documentation there is also stuff (I think) in the repository of the legacy part of the website which is not only ugly but also almost impossible to understand and maintain. But to be honest I don't really know h...
I think nobody will be able to tell you why it is this way as this comment has been written 8 years ago (i.e between php 3.0 and php 4.0 release), that's pretty old ^^ About the performance issue, it's the same thing: it was an issue 8 years ago but PHP changed a lot since then and it's not an issue...
Actually phpBB template engine IS Twig with an overlay for BC and historic reasons. So you can just use the regular template service and you can assign arrays and objects like with Twig (unlike the 3.0 template engine)
The class loader is registered in the container for some obscure reasons but it shouldn't. The has to be the first thing created and that's a requirement you cannot change. About what you are trying to achieve, the best thing is to inspects the autoloaders chain and wrap them (give Symfony's Debug a...
Actually what you are trying to achieve is just impossible. The container is compiled once per cache refresh and after that you cannot alter it (and or remove any service definition). It is possible to manipulate the container builder during the compilation phase but you don't have access to the dat...
I made a mistake indeed, "phpbb/phpbb": "~3.1.10|~3.2.0" is the best one (not sure about "phpbb/phpbb": ">=3.1.11,<3.2.0|>=3.2.1,<3.3.0", it should work I think, but...)
You can still upload it somewhere and give me a link (together with a link to your forum if it's public so I can see what it does^^), it can be a great help and source of inspiration if you don't have the time or motivation to contribute them directly to the core.