Login to vB From External PHP Page on Different Domain

[Jennifer Marx]

I've been searching for a solution to this for several years now, and thought perhaps I should ask here. My vB 3.8x installation is on http://www.passporterboards.com/forums. I am able to use global.php on other, non vB pages on the http://www.passporterboards.com domain to allow my users to login. But what I really need is to allow my users to login on a PHP page on http://www.passporter.com, and stay logged in. I know it's technically possible -- my PhotoPost installation is on passporter.com and it can login users to vB and keep them logged in. I have access to the vB database on both domains, as they are on the same server. I understand that the reason I can't just use global.php on passporter.com is a cookie issue. But there must be some other way to do this. ANY hints, leads, clues, or ideas are greatly appreciated. Thank you!