include("http://www.aaa.de/test.htm"); <= geht nicht
As long as support for the "URL fopen wrapper" is enabled when you configure PHP (which it is unless you explicitly pass the --disable-url-fopen-wrapper flag to configure (for versions up to 4.0.3) or set allow_url_fopen to off in php.ini (for newer versions), you can use HTTP and FTP URLs with most functions that take a filename as a parameter, including the require() and include() statements.
Note: You can't use remote files in include() and require() statements on Windows.
[Aus: PHP3-Anleitung/features.remote-files.html]