mysql - Codeigniter PHP url segments for users profile -

I need that when I type domain.com/user/1, pull one as a variable And then can be used to get the right database value. Now the variable is set manually with the domain. How do I set up this segment and then get that number? I am just working with the number of username URLs, so the number, just domain.com/user/1, domain.com/user/487 ect