Outputs or save an XBM version of the given
image.
imageAn image resource, returned by one of the image creation functions, such as imagecreatetruecolor().
filenameThe path to the saved file. If not set or NULL, the raw image stream will be outputed directly.
foregroundYou can set the foreground color with this parameter by setting an identifier obtained from imagecolorallocate(). The default foreground color is black.