Check the redirect url

[eluser]Iwasignited[/eluser]
Hi everyone, I need your help again.

Im building a weblog with CodeIgniter.
In the entry page, there is a link to add comment page.
For example: In mysite.com/entry/1, there is a link to mysite.com/add_comment/1.
Now, I wanna check the redirect url of the url.
When user goes to add_comment/$id from entry/$id, its ok.
But when he types add_comment/$id in his address bar or goes to the add comment page from another site, he will see an error page.
Im wondering how can I do that.

Thanks for reading and sorry for my English grammar/vocabulary.

Looking forward to a solution.
Thanks everyone.

[eluser]danmontgomery[/eluser]
Check the referring url.

Code:

$_SERVER['HTTP_REFERER']

http://www.php.net/manual/en/reserved.va...server.php