How to log in to a third-party site in php through vk?

Question

PHP

I am writing a PHP project that requires authorization on the vk.com site, I log in using direct authorization provided by the VKontakte site itself. Authorization goes without problems and working with api also does not cause trouble. But it was required to log in to a third-party site through the vk.com site. So here is the problem. When you access the required site through a browser and click log in, a window opens . How to log in there, or rather, transfer the information that I already have a token and all the information for working on Vkontakte.
Since messing around and sending requests through curl is not very much a hunt.

Answer 1

And how do you imagine yourself without curl?