how can i use fopen() with CodeIgniter?

[eluser]WanWizard[/eluser]
This has nothing to do with CI, the webserver is returning an error code.

I find it hard to believe that if you put those two lines in a stand-alone PHP script it works, and from a CI application it doesn't.

Or do you mean that 'www.example.com' is a CI application? You're not very clear on that. In that case, error 500 is probably a rewrite error.