imagecopymergegray() copy a part
of src_im onto dst_im starting at the x,y
coordinates src_x, src_y with a width of src_w and a height of src_h. The portion defined will
be copied onto the x,y coordinates,
dst_x and dst_y.
The two images will be merged according to pct which can range from 0 to 100.
When pct = 0, no action is
taken, when 100 this function behaves identically to
imagecopy().
This function is identical to
imagecopymerge() except that when merging it
preservese the hue of the source by converting the
destination pixels to gray scale before the copy
operation.
注: This function was added in PHP 4.0.6