Can I ask questions on Stack Overflow about how to learn creating a CMS with PHP/CodeIgniter?
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No, I'd say such questions too quickly devolve into a shopping list recommendation and/or are too vague and overly broad.
Instead, while learning, try out different pieces of code and ask questions about the concrete problems you face with those focused tasks.
It could be that 'how do I learn tech X' questions are on-topic at Programmers, but do check their FAQ first before you ask.
It's clear in FAQ panel about what type of questions one can ask on Stack Overflow: