I just learned a thing about moving a phpBB 3.3 installation to a different domain that's not in the Knowledge Base article about it, and I wanted to offer my input to help others. I'm referring to this article:
https://www.phpbb.com/support/docs/en/3 ... or-domain/
The problem I had was that after I followed the article step by step and successfully moved (copied) my phpBB install to the new domain, it kept redirecting to the old domain on login.
The solution was to empty the /cache folder.
I didn't see this in that article, but I did find it elsewhere, and I'm wondering if it should be added. 
I was actually setting up a copy of my phpBB install on my local server for development and messing around purposes.
If anyone needs help with this or anything at all, please feel free to ask me.