In case you are using MySQL, a not-so-sexy yet often adequate solution is to use phpMyAdmin as a rudimentary admin interface. With phpMyAdmin you can get things going, and later on, when really in a need of one, you can use your PHP framework's capabilities to generate a nicer admin interface.
It may also well be that you are actually not looking for an admin GUI, but rather for somekind of a CMS. For instance Drupal is a PHP-based CMS that allows you to create all kind of custom content. It also readily provides administration and content creation interfaces out-of-the-box.
Can you specify in a little more detail: who is going to use the admin interface? Are all end-users going to use it? Or do you share admin accounts only for several managers/webmasters? Also what kind of data repository are you using?
If you want a more specific answer, please provide more details on the question (see above).