title: URL Friendly Username in PHP?
body: <p>On my PHP site, currently users login with an email address and a password.  I would like to add a username as well, this username they g\set will be unique and they cannot change it.  I am wondering how I can make this name have no spaces in it and work in a URL so I can use there username to link to there profiles and other stuff.  If there is a space in there username then it should add an underscore jason_davis.  I am not sure the best way to do this?</p>