Ask the question what will it cost you if there is a collision. If this is a free site fine. If you are running a money making business and an overrite will cost you a million dollar contract then I would think again.
I think you are going about this the wrong way.
I think you need to keep the unique ID but you want to make sure that the users can not manually change the ID.
One way to to do this is to put the ID and the hash of the ID (with some extra data) in the link.
For Example: (my PHP is rusty so general algorithm would be:)
id = 5;
hash = hash("My Private String " + id)
link = "http://mySite.com/resource?id=" + id + "&hash=" + hash
Then when you receive a request just validate that you can regenerate the hash from ID. This does leave you open to an attack to work out "My Private String" but that will be quite computationally hard and you could always append something else unique that is not directly available to the user (like the session ID).
