if your users are predominantly using internet explorer you can take advantage of the following. Although I am running a intranet site using ASP you can google to see if you can capture the same with PHP.
IE can grab the username from windows OS, see example below.
<%
dim user_name, user_name_parts, authuser
' get the username from the client machine
authuser = Server.HTMLEncode(Request.ServerVariables("AUTH_USER"))
' split the username string
user_name_parts=Split(authuser, "\")
' return only the username
user_name = user_name_parts(1)
%>
