How to post data in PHP using file_get_contents?

I'm using PHP's function `file_get_contents()` to fetch contents of a URL and then I process headers through the variable `$http_response_header`. Now the problem is that some of the URLs need some data to be posted to the URL (for example, login pages). How do I do that? I realize using stream_context I may be able to do that but I am not entirely clear. Thanks.