imagecolortransparent() sets the
transparent color in the
image image to
color. image is
the image identifier returned by
imagecreate() and
color is a color identifier returned by
imagecolorallocate().
注: The transparent color is a property of the image, transparency is not a property of the color. Once you have a 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.