Image Alpha Channels

Thanks, in advance. PHP's new image functions contain alpha transparency. How does one go about setting a color which has a transparent quality? For instance.. I would like to create two rectangles. one black, one grey, on a white background. I would like the grey one to be on top, and be 50% transparent, so that the black one will show through. Is this not possible? I can't seem to figure out how to create the "grey" color with an alpha channel property, so that it will allow the black box to show through. Best wishes. Michael Richardson "phpzen"