Originally Posted by Jim

Make sure you don't ouput (eg: echo, or even just whitespace) anything before calling the Excel functions. This error means that the options (headers) of the page have already send while your Excel class tries to add/change a header. After the headers are send, you can't edit/add headers.

You can fix this by simply calling ob_start(); at the top of your script, not a very clean way to fix it though...