i gess it is impossible, cause if you call for http://... you are asking for the content of the file for apache (or whatever server soft) and it will be interpreted before send for you.
well, otherwise the include function on php, does not apper to suport remote include (look at php.net/include)..
i would sugest (from php.net/file)
$fcontents = implode ('
', file ('http://adress/file.php.txt')); // you need to use some extension that doesn't be interpreted by apache..
// so..
eval($fcontents);
// that point you included the file.php.txt
