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I can recommend this oAuth library for PHP to implement Twitter sign, it's very minimal so it's easy to integrate.

What's the site written in? If it's rails, check out twitter-auth: http://github.com/mbleigh/twitter-auth

I would recommend: http://github.com/jmathai/twitter-async/ - it implements Twitter's OAuth API. Will save you some work.

I can vouch for Twitter-Async.