phpBB extension that uses jQuery to force external links to open in a new window when clicked on.
Clone (or download and move) the repository into the folder ext/rmcgirr83/elonw:
cd phpBB3
git clone https://github.com/rmcgirr83/elonw.git ext/rmcgirr83/elonw/
Go to admin panel -> tab customise -> Manage extensions -> enable External Links in New Window