PHP - Generate transparency (or opacity) gradient using image

Using PHP is it possible to take an existing image and produce a copy where the original image changes from 100% opaque开发者_开发百科 to 0% opaque (or 0% transparent to 100% transparent!)?

Thanks,

Toby

EDIT: I realise my question was unclear, apologies. I want to take an image and create an opacity gradient across it, so the image is 100% opaque at the top, 100% transparent at the bottom and translucent in between.

If you change from 100% opaque to 0% opaque the image wouldn't be visible anymore. What do you want to do exactly? Change The whole Image or only a part? I would consider of using CSS (maybe + javascript) where you are able to set the transparency between 0% and 100%.

Well if you take an image from 100% to 0%, it disappears right ? and 0% to 100% means it was transparent so it went black? I'm not sure exactly what you are asking bu try this tutorial