On the SitePoint PHP blog there's a new post from Lukas White showing you how to use the Picturefill plugin (Javascript) along with PHP to make responsive images.

One of the key challenges with responsive web design, and a subject of much discussion in recent years, is how to deal with images. Setting a max-width on image elements enables designers to allow their size to adapt to the page dimensions, but in itself that approach can lead to far bigger images being downloaded than are required. [...] You can use a similar approach [to "source sets" of images] straight away and in a cross-browser compatible manner by using Javascript; one such method is the Picturefill plugin. In essence, Picturefill allows you to specify different src attributes for an image, each image file corresponding to a different media query. Thus

The tutorial helps you create an application, powered by the Slim framework and the ImageMagick extension, for the basic structure. He then grabs the Picturefill library and drops them into place. Some sample code is also included showing how to create the HTML structure for the images and the Javascript to handle the switching.