p01-contact : simply add contact forms in your pages

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How to access various information of GS, such as description, …

When a page is loaded to be displayed a variable $metad is created with the description of the page.
Running something like the following function when a page is being loaded should be able to change the description the way you want it.

Code:

function rewriteDescription() {
   global $metad;
   $metad = preg_replace(); # Strip away your tag here.
}

… language or …

What language do you want to check? The one used in the admin panel? It is stored in $LANG.

email administrator?

Probably at $datac->EMAIL. Please note I’ve never used this one so I can’t be sure.

Of course all of the above is only true when other plugins haven’t reused these variables in the global scope.

And is it possible to reuse the pretty error message used by GS?

Those are all printed by the /admin/template/error_checking.php file. There are too many different ways to get into right now in a reply so I advice you to take a look at what variables might trigger error_checking into working.