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.