The solution!!
Thank you all for your kind and valuable answers.
I was a bit afraid of using COM or other "foreign" technologies to integrate it with my php.
The $_SERVER['LOGON_USER'] works fine, except that I needed anonymous user access for the rest of the intranet.
So, I decided to go for the lazy, easy way. I simply created a virtual directory within my intranet and removed the anonymous user from that directory. Now, I can user the $_server... variable with impunity and pass, if need be, to the other script. Or I can write my script on this area.
Many thanks to all of you.
All the best!
Lubatico