Currently there is no way to replace template code in order to achieve what MODX files did with the replace-with and inline-replace-with commands. So it is impossible, to say, add a new class to an existing line of template code, or simply replace an entire chunk of template code with a new chunk.
The template engine supports TWIGs block tags, usingwhich in theory would allow any template code wrapped in BLOCK tags in an extension to overwrite BLOCK tags of the same name in the core.
1. There are no BLOCK tags in the core
2. It doesn't work: TWIGs block syntax only works in template inheritance, so it only works when a template is created using
So phpBB needs a means to allow template code replacement (not just injection)