PHP page Variable Redirect based on User Login

I am trying to make a php page that would redirect to a static url based on the user login name of the user. For instance, if their login name was “john” it would redirect them to http://www.domain.com/john.

Any ideas? I’ve tried various things and can’t get anything working and am getting a lot of “call to undefined function” problems. Plus I am not real skilled in php.