Let an administrator login as user

I want to be able as an administrator to login as a user. At the moment, all my users need to enter their email and password to login. The password is encrypted with md5 and a salt key. How can I do this?

EDIT

I have a service where users advertise their holiday homes. I want to login as user in order to correct some problematic info they enter. I can do this from the administration page, but I thought that logging as a u开发者_如何学JAVAser would be faster to edit.

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I would say build a page that is only accessible to administrators, and allows the user to change his uid in the session variable, or the cookie, or wherever you are storing that info.