Image segmentation and Image Recognition - How to recognise the coordinates of a white patch in a black background image?

Aim: Find the white patch with the biggest area. And if possible then confirm if it is a circle. Then find the average centre of this patch and radius.

I have thought of creating an edge around the images, but I don't know how that would help in finding out the area of the patches.

I am trying to accomplish this and if anyone can guide me in the right direction, I would really appreciate it.

I am currently using php.gd library and that is the only tool I have.

Thanks