It's fine to use the images, but as stated on the sprite pages if you're hotlinking them you should link back to this site as well. However I'm not clear on what you mean by "simple php requests", surely you mean using HTML like an image tag? <img src="...">
etc.
As others suggested you can also add the images to your own site. We don't have bulk downloads here (so you'd have to get them one-by-one) but there are plenty of places to find downloads, like veekun.com. Hope that helps.