After more research I discovered for jsonp callbacks to work, the PHP needs to return the name of the function as well as (which the flicker service does in the $.getJSON example), so I added this to the PHP file. For reference the PHP file now consists of this:
<?php $stuff = "myCallback ({'prop':1)"; echo $stuff; ?> However, the page is still not working! Firebug now reports the error 'myCallback is not defined', but I thought jQuery was supposed to using the jsoncallback=? query parameter? I guess this must be a problem with the PHP? Does no one know this or am I just posting in totally the wrong place?