(a): The limitation on the size of a URL (querystring included) is more like 128 characters, not 128kB...
(b): Why not have inline.php get the string from the database itself? OR (and this one smells a bit dodgy to me for some reason), store the string in a session variable for the frame src to retrieve when it loads.
(c): GET and POST (and, allegedly, PUT) are the different types of requests an http client can make to an http server. Details are beyond the scope of this thread; consult any decent http reference (such as that at the W3C).