Description
int 
imagecolortransparent ( resource image [, int color] )
    imagecolortransparent() sets the transparent
    color in the image image to
    color.  image is the
    image identifier returned by imagecreatetruecolor() and
    color is a color identifier returned by
    imagecolorallocate().
   
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.
    
    The identifier of the new (or current, if none is specified)
    transparent color is returned.