Access functions.php from another server?

Hi everyone!  I am working on integrating several other sites(a store and a travel section) which are hosted by other companies on their servers, to my own.  I have ran into a bit of a snag on the store site.  I am able to cut and paste header code, body code, etc... to customize the store on their server to look exactly like my dolphin site.  THis is my problem.  How can i access teh FUNCTIONS.PHP of my dolphin site from the other site?  I want memebrs to be able to login to my dolphin from there - and more importantly - after logging into dolphin, once they click the STORE link, i want it to show that they are still logged in at the top.  IDEAS???

I am drawing blanks.......

.php top frame

.php body

like a exit script. that holds users in your site while on other sites

something like click farms is the best I can come up with.

well hope that helps some.

take care Jeremy & Ryver