Facebook PHP SDK: getting “long-lived” access token now that “offline_access” is deprecated

Question

BASIC PROBLEM: I want my app to be able to make calls to the Facebook graph api about authorized users even while the user is away.

For example, I want

Answer 1

The selected answer is now outdated. Here are Facebook's instructions to swap a short-term token (provided in front-end) for a long-term token (server only):

https://developers.facebook.com/docs/facebook-login/access-tokens/refreshing/

Generate a Long-lived User or Page Access Token
You will need the following:

A valid User or Page Access Token
Your App ID
Your App Secret
Query the GET oath/access_token endpoint.

curl -i -X GET "https://graph.facebook.com/{graph-api-version}/oauth/access_token?  
    grant_type=fb_exchange_token           
    client_id={app-id}&
    client_secret={app-secret}&
    fb_exchange_token={your-access-token}" 

Sample Response
{
  "access_token":"{long-lived-access-token}",
  "token_type": "bearer",
  "expires_in": 5183944            //The number of seconds until the token expires
}