imagejpeg() creates the JPEG file in filename from the image
image. The image argument is the return from the
imagecreate() function.
The filename argument is optional, and if left off, the
raw image stream will be output directly. To skip the
filename argument in order to provide a quality argument just
use an empty string (''). By sending an image/jpeg
content-type using header(), you can create a PHP script that
outputs JPEG images directly.
注: JPEG support is only available if PHP was compiled against GD-1.8 or later.
quality is optional,
and ranges from 0 (worst quality, smaller file) to 100 (best
quality, biggest file). The default is the default IJG
quality value (about 75).
If you want to output Progressive JPEGs, you need to set
interlacing on with imageinterlace().
See also imagepng(),
imagegif(), imagewbmp(),
imageinterlace() and
imagetypes().