A lot of sites now are allowing account registration and login using their existing Google/Facebook/Twitter accounts. Integrating this sort of login is going to totally change your user registration system. There are the external APIs to handle. Then there are all the account management sections and UI/account issues such as users accidentally registering again, linking existing accounts to multiple Google/Facebook/etc accounts, etc.
Yii has the LOID (LightOpenID) extension available for this sort of OpenID login, or if you need a full login system, there is yiiauth which includes this functionality.
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