Hmm, this is not an answer to your question but I want to share it anyway.
I have used Notepad++ for years. I didn't like Eclipse and NetBeans is just so damn slow.
I never used the auto complete function for anything.
Notepad++ can't really do any code analysis, it can only give you auto-completion on the known PHP functions.
I've since switched to an IDE that does full code analysis and can do auto-completion for your own code as well.
Notepad++ is great if you just want to write something real quick. It's also a great tool if you want to learn about PHP since it doesn't do auto-completion very well ;) but if you really want to be productive when you're working on a large application I recommend you use an IDE that does code analysis. Productivity is the key here. I spend less time looking through files and more time actually writing code, which is great! :D
Good IDEs for PHP: NetBeans, PHPStorm, Eclipse.