php image manipulation: masking

[HaLo2FrEeEk]

Does anyone know of the top of their head if there is a function built in to php that will allow me to apply image masks to an image?  If you don't know what a mask is, it is a black or whitelayer on an image that is interpretted into either opaque or transparent.  I need to take a base image and cut it out on the fly (the image will be different every time in style, and color) leaving just a certain part of the image, the rest will be transparent.  If there is no built in function, then can anyone refer me to a class created for this purpose?  Thank you.

[Barand]

Well, if you have, say, an image and your mask image is same size but with white background with a blue circle in the middle (the mask)

 

pseudocode

for x = 0 to imagewidth
   for y = 0 to imageheight

         if pixel at xy in mask is white then
             output white pixel in output image (or other transparent color)
         else
             copy pixel color from image at xy to output image
         end if
   end for
end for

[HaLo2FrEeEk]

That would take forever though, going through every pixel.  I'm looking for something a little faster and less straining on the server.  The base picure I'm working on is a little too big to go through pixel by pixel.