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 preserves the hue of the source by converting the destination pixels to gray scale before the copy operation.
Nota: This function was added in PHP 4.0.6