You seem to be going about this in a very wrong way.
What include() (and require(), for that matter) does is that it parses the specified file with the PHP interpreter. If your aspx-code for some reason generates PHP-code, which is supposed to be parsed by the PHP interpreter, then the way Paul Dixon suggests would be the right way of going about this. However, I would strongly advice against doing this.
For one thing, it's a huge security disaster waiting to happen. It's also incredibly bad architecturally speaking.
If you want to include HTML markup etc. generated using aspx, what you should do is to use
echo file_get_contents("http://www.example.com/Default.aspx");
This way, any PHP code in the output remains unparsed, thus avoiding the aforementioned security disaster. However, if you're able to do without mixing languages like this, that's probably going to be a much better solution.