Thread:Extension talk:ConfirmEdit/Fatal error when I try "Log in / create account"/reply (7)

You need to use the REL1_18 branch for both your main MW install and this extension. You can fix this by going to your ConfirmEdit directory and running:

svn switch http://svn.wikimedia.org/svnroot/mediawiki/branches/REL1_18/extensions/ConfirmEdit

Apparently, the change with getIP hasn't been propagated to the REL1_18 branch.