PHP Facebook SDK.About the access Token - php

Question

Now I can get my own information such as my id,username.But when I use this access Token in order to get my friend lists,then the error is :
An active access token must be used to query information about the current user.
What is this mean?why the access Token doesn't work?
the first php:
$permissions = ['email']; // Optional permissions
$loginUrl = $helper->getLoginUrl('http://my_ip/facebook/login-callback.php', $permissions);
the second php:
(I can get my information)：
try {
$response = $fb->get('/me');
var_dump($response);
echo "<br>";
$userNode = $response->getGraphUser();
} catch(Facebook\Exceptions\FacebookResponseException $e) {
// When Graph returns an error
echo 'Graph returned an error: ' . $e->getMessage();
exit;
} catch(Facebook\Exceptions\FacebookSDKException $e) {
// When validation fails or other local issues
echo 'Facebook SDK returned an error: ' . $e->getMessage();
exit;
}
//echo "<br>";
echo 'logged in as the id '.$userNode->getId()."<br>";
echo 'Logged in as ' . $userNode->getName()."<br>";
echo 'the user picture is '.$userNode->getPicture()."<br>";
$picture = $userNode->getPicture();
and:
$permissions = ['user_friends'];
$friendsUrl = $helper->getLoginUrl('http://my_ip/facebook/friends-callback.php',$permissions);
finally,in the third php,I use the same way to get the information,but it is failed.

Answer 1

I think you missunderstood the concept of Facebook API.
First you have to get user permission to accept your request:
$permissions = ['email', 'user_friends']; // Permissions
$loginUrl = $helper->getLoginUrl('http://my_ip/facebook/login-callback.php', $permissions);
Then, in your login-callback.php file you can access the API according to requested permissions. For friends list:
$response = $fb->get('/me/friends');
var_dump($response);
You don't have to login every time after yout first request. The access token will be saved in session.