Find The Least Common Characters

Since you included the tag of "postgis" I assume that is one of the software/database package's you are interested in.

The book "PostGIS in Action" was just finalized, printed and started shipping about a week ago. At $50, it's not a bad price. It appears to be quite comprehensive @ 520 pages, and could be a valuable reference guide for anyone interested in PostGIS and its various components. You can download the first chapter for free, which also includes a brief table of contents to give you an idea of what it covers.

The easiest way would be to check for an stub get_option something like

$run_once = get_option('run_once');

if (!$run_once){
    //show your custom message here
    // then update the option so this message won't show again
    update_option('run_once',true);
}

hope this helps