I have a OAUTH 2.0 Server, and i have succesfully implemented it up to the last level using PHP

Reference Link : https://developers.google.com/actions/identity/oauth2-code-flow

However, im am on the Last Step now which says

---> Return the following JSON object in the body of the HTTPS response

 {
 token_type: "bearer",
 access_token: "ACCESS_TOKEN",
 refresh_token: "REFRESH_TOKEN",
 expires_in: SECONDS_TO_EXPIRATION
 }

I have done the following

$json = array(
'token_type:' => "bearer",
'access_token:' => $ACCESS_TOKEN,
'refresh_token:' => $REFRESH_TOKEN,
'expires_in:' => '3600'

);
  $jsonstring = json_encode($json);
  echo $jsonstring;
  die(); 

In which $ACCESS_TOKEN and $REFRESH_TOKEN and two variables containing the necessary information.
My Question is how do i return the 'JSON object in the body of the HTTPS response: '?

Would mean a lot if anyone can help me figure this out
Thanks in Advance!
Please visit this site, and see you might understand what i'm trying to do then :)
https://developers.google.com/actions/identity/oauth2-code-flow