image2wbmp() creates the WBMP file in filename from the image
image. The image argument is the return from
imagecreate().
The filename argument is optional, and if left off, the
raw image stream will be output directly. By sending an image/vnd.wap.wbmp content-type using
header(), you can create a PHP script that outputs
WBMP images directly.
注: WBMP support is only available if PHP was compiled against GD-1.8 or later.
See also imagewbmp().