Well that pretty easy. In the code for checking the users login you can simply use a redirect header.
After the login has been verified you can grab the username either from the form they submitted or from the query you did to validate the login.
$username = $_POST['username'];
$username = $row['username'];
Obviously the exact coding will depend on what your coding is.
And then all you need is to redirect them with a redirect header like so.
header("Location: /members/" . $username);
you can use either a relative path something like above or use the full URL.
header("Location: http://www.mysite.com/members/" . $username);
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