Those are dynamic functions- in other words it’ll say “register/site admin” and “log in/log out” depending on whether you’re logged in or out.
Haven’t tried php functions in a text widget, but what better time to try than now? If that doesn’t work, you can always search for a meta-widget in the plugins here, and then open it up and tinker in the .php file.
The code for the widgets are in wp-includes/widgets.php
But I wouldn’t mess with that unless you really know what you’re doing.