Page-by-page authentication
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http://blog.epistemographer.com/?p=238
A variation of the code above for group based page-by-page authentification can be found here
In case you get a parser error while using this plugin, try changing the echo command at the bottom like this:
echo "<body onLoad=\"javascript: alert('Not Allowed')\">";
Also for installation, instead of adding line include("extensions/accessControl.php"); to your LocalSettings.php as specified by the author use require_once("extensions/accessControl.php");
Note: This extension is to restrict read access to a page. This is not for restricting edit access.
A new extension has been forked from this one, which now restricts edit access also, including group-based authentication.

