Convert a JPEG to a grayscale JPEG using PHP and the IMG_FILTER_GRAYSCALE type of filter.

header("content-type: image/jpeg");
$JpgImg = imagecreatefromjpeg("Central Park.jpg");
imagefilter($JpgImg,IMG_FILTER_GRAYSCALE);
imagejpeg($JpgImg);
imagedestroy($JpgImg);