NT Domain UserNames

I have a critical problem: I have an intranet built using PHP and SQL server residing behind an NT domain, and only the people who have access to that NT doamin can view this site. Using IIS Authentication types, I have been able to make the site password protected and the any user with the username/password for that NT domain can log onto the site using the same username/password that they entered for the Domain Authentication. However, the issue is that They have to enter the username/password twice: Once when they enter the NT domain and second when they enter the website. My question is: Is there a way to integrate the Network username/password into the website without telling them to re-enter it everytime they try to access the site for the first time ? Any help would be greatly appeciated. Thanks, Vishal