IMHO manual to formułuje bardzo jasno:
Cytat
If "URL fopen wrappers" are enabled in PHP (which they are in the default configuration), you can specify the file to be included using a URL (via HTTP or other supported wrapper - see Supported Protocols and Wrappers for a list of protocols) instead of a local pathname. If the target server interprets the target file as PHP code, variables may be passed to the included file using a URL request string as used with HTTP GET. This is not strictly speaking the same thing as including the file and having it inherit the parent file's variable scope; the script is actually being run on the remote server and the result is then being included into the local script.
Czyli includowany plik ze zdalnego serwera jest odpalany na zdalnym serwerze i to co zwróci jest includowane do lokalnego skryptu. Nie ma więc dostępu do funkcji i zmiennych.