Sets the transparent color in the given image.
imageAn image resource, returned by one of the image creation functions, such as imagecreatetruecolor().
colorA color identifier created with imagecolorallocate()
The identifier of the new (or current, if none is specified) transparent color is returned.
Note: Transparency is copied only with imagecopymerge() and true color images, not with imagecopy() or pallete images.
Note: The transparent color is a property of the image, transparency is not a property of the color. Once you have set a color to be the transparent color, any regions of the image in that color that were drawn previously will be transparent.