Showing a message to new users

To show a message to new users (for example rules or information about the community or a warning that messages will be moderated) use following code in overall_header.html. (ACP>Styles>Templates>overall_header.html)

This will first check if the user is logged in and then check if the user has made no posts yet. If both is true, the text/HTML you put in the place of will appear on every page (since you insert the code in overall_header.html).

Usage example
As a user of phpBB 3.0.5 I'd recommend adding the following code to the bottom of overall_header.html.

{L_INFORMATION}:

Credits
irc.freenode.net #phpbb-coding and http://wiki.phpbb.com/Template_Syntax#User_Variables were helpful at creating this information.