I want an easy proxy PHP function/script that may download personal files from the foreign domain, as well as let me customize the data (as strings) before retransmit. Any ideas much better than "readfile($url)" ??

file_get_contents(), fopen(), http_get() or curl are techniques and processes you most likely wish to have a look at. Based on what you're installing and just how you need to change it out you could utilize XML Parser, regex or perhaps a simple str_replace().