Description
int 
imagecolorexactalpha ( resource image, int red, int green, int blue, int alpha )
    Returns the index of the specified color+alpha in the palette of the
    image.
   
    If the color does not exist in the image's palette, -1 is
    returned.
   
Note: This function requires GD 2.0.1 or later.
    See also imagecolorclosestalpha().