PHP Face Detector is a library based on PHP and javascript that allow you to perform face detection operations on images.
More than that, the javascript API delivered with it provides useful methods and events that you can use to build features like facebook has: place detected faces on image or place tagged persons.
The face recognision algorythm is an efficient one. It allows you to use any size of image. It won't load your server useless. All computing operations are made server-side, and the recognised faces details are taken though AJAX by the javascript library, which will draw the squares around.
It has also great styling flexibility. All squares around the detected faces and the container that holds them can be styled as you want by editing the dedicated css file.