Open-source CMS Drupal has a PHP mode: in addition to HTML (with or without restrictions on available tags) suitably permissioned users can put PHP in the box, enter it into the database and have it executed (eval()'d, in particular).
At first glance, this is really useful. After some more thought, although you might be tempted to throw in the odd 'echo date();', running worthwhile scripts within the environment of your CMS is going to have unnecessary overhead, even assuming you can get the two to coexist peacefully.

As regular readers may know, I'm something of a glutton for punishment, so I tried it anyway.

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