Redirect after login to URL with variable
 

I have been digging for days in these forums and getting close but can't seem to get there. I have an external form for logging into our Vb5 site. I need to redirect after login to our home page which is accomplished easily by including the hidden [url] variable from the form. Unfortunately, I can't do it that way as the URL I need to redirect to must contain the userid for the Vb5 logged in user (https://homepage.asp?u=[userid] or example).

I expect I could capture the userid either somewhere in the login process (login.php or auth.php) and execute the redirect from there or on the home page once the login process has completed then execute the redirect from there. The problem is I am very new to PHP and am not sure of 1.) How to capture the userid
2.) The syntax for the redirect with the variable
3.) The Vb5 page to place it on

Any help would be greatly appreciated!!! In fact, you may save a poor man's sanity. Thank you!