Maurice Svay has a new blog post that includes a script he's developed to perform facial recognition (detect faces in images) with PHP without the need of the Open-CV library.

OpenCV seems to perform well but you need to be able to install it on your server. In my case, I wanted to have a pure PHP solution, so it can work with most hosts. So I started to think about implementing it myself. [...] I kept searching and finally found a canvas+javascript implementation of face detection at http://blog.kpicturebooth.com/?p=8. The code looked fairly compact and simple. Shouldn't be hard to port to PHP.

The class takes in the filename of an image (just JPG, but could easily be adapted) and a data file to use to run the image through the GD image library and output a JPG similar to this with the face highlighted by a red square.