How to check if a user has joined the Vk/Facebook/Ok community using the API?

Question

PHP

We plan to reward users who will join certain communities. To do this, we need to understand whether the user has joined the desired community. The assignment will be posted on the site. There will be no authorization through the site. We need some kind of solution that, through the API, will allow us to get a response about whether the user has joined the community or not.
1. Tell me, what are the solutions?
2. What will be the user ID and how can we check the fact of joining the community?
3. If there is an example of the implemented code, I will be very grateful.

Answer 1

VK: https://vk.com/dev/groups.isMember

Answer 2

It's called Social Lock, social. networks are not welcome. Usually a button is made like "Get a coupon through VK", the user clicks on the button, then he installs your VK application for himself (authorizes through it), then you have a user ID and token and you check the user's membership in the community. If there are alternative ways to get a user ID (clickjacking), but they are punished very severely by both search engines and social networks. networks.
In general, the practice of Social Lock is doubtful, I strongly advise you to abandon it.
How can you make sure that the user still joins the community.
Option 1. "Cunning-ass". You have a closed community, in which you post coupons, as soon as a user joins it, he gets access to coupons with discounts. Some users leave the community immediately after joining and gaining access to coupons.
Option 2. "Correct". Your communities are interesting. They have coupons, articles, good content about the products you sell. For example, makeup and cooking lessons. Reviews, hacks, jokes work for men. Those. your communities are professionally run. This is an expensive option, but works well with decent offices. People often join such communities and sit there for years, inviting friends to them. The site usually integrates with them by embedding posts on product pages and widgets.