How to check 404 URL errors?

[eluser]FutureKing[/eluser]
Hi,

I am developing a social bookmarking application with codeigniter.
When user submits any url I want to check that is that url exists?
I mean I want to prevent wrong URL submissions.

One more question:
I also want to check the request type(i.e. POST or GET or normal)
in ASP.net I use

if Page.ispostback

Is there something similar to it available with PHP or code igniter?