not sure where but I came across as image hosting site that allows you to upload your image in a large format or to sharpen it.
I personally do not recall or know of any functions of GD library to sharpen image which might just be a different word for quality being up-ed.
If some one knows of a function to sharpen images please tell me since personally I am not aware of such functions neither in Image Magic and/or GD Library.
You'd use imageconvolution() with a matrix that corresponds to a sharpen filter. Details in this comment on the PHP site: http://php.net/manual/en/ref.image.php#56144