Question

How do I find at what time an access token is going to expire in php?

Answer 1

The token you receive initially from Facebook in the signed_request expires in 2 hours or 7200000 milliseconds or 7200 seconds. If you extend the token with the below request you will receive a new expire time of 5184000 seconds in the response which converts to 60 days. What I typically do is store this time in milliseconds added to the current Unix time in milliseconds since the epoch and continually check against that time when needed.

Extending the token:

https://graph.facebook.com/oauth/access_token?             
    client_id=APP_ID&
    client_secret=APP_SECRET&
    grant_type=fb_exchange_token&
    fb_exchange_token=EXISTING_ACCESS_TOKEN 

As an example I would request that sources information on the client side and do the following check..

if (response.user.sources.FACEBOOK.expires > new Date().getTime() ) {
    //do something
}