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?