During my website everyone can send some links with other nice websites. All links during my database must by unique, however, many links are with 'www.' prefix, plus some without. Some finishes for '/', some not. For instance:

http://world wide web.example.com

http://example.com

http://example.com

http://example.com/

along with other problems could be with https or http.

I understand which i should change address before saving to database, but what standard I ought to use?

Well, you cannot always treat http://world wide web.example.com and http://example.com because the same site, simply because they could offer different content (even though it would break lots of peoples' anticipation)

Similarly http:// and https:// addresses should not be looked at to suggest towards the same content. When the server was setup properly, duplicate Web addresses might have a canonical redirect established to point together. When the server is not setup properly it will likely be tough to tell if the duplication is as simple as design or accident.

A great way is always to follow any URL you are given and find out whether it redirects to a different. No matter what, make use of the URL you finish up at after any redirects.

I'd use http://domain.com. Wether standard you select, just stick to it throughout your code.

I believe the least one, for instance domain.com. But when you've ftp://domain.com, you have to supplment your DB additional protocol posts.