Get the FB access token set by the JS API

get_facebook_cookie.php

private function get_facebook_cookie() {
	if (isset($_COOKIE['fbsr_' . $this->fb_config['appId']])) {
		list($encoded_sig, $payload) = explode('.', $_COOKIE['fbsr_' . $this->fb_config['appId']], 2);

		$sig 	= base64_decode(strtr($encoded_sig, '-_', '+/'));
		$data 	= json_decode(base64_decode(strtr($payload, '-_', '+/')), true);

		if (strtoupper($data['algorithm']) !== 'HMAC-SHA256') {
			return NULL;
		}
		$expected_sig = hash_hmac('sha256', $payload, $this->fb_config['secret'], $raw = true);
		if ($sig !== $expected_sig) {
			return NULL;
		}
		$token_url 	= "https://graph.facebook.com/oauth/access_token?" . "client_id=" . $this->fb_config['appId'] . "&client_secret=" . $this->fb_config['secret']. "&redirect_uri=" . "&code=" . $data['code'];
		$response 	= @file_get_contents($token_url);
		$params 	= NULL;
		
		parse_str($response, $params);
		
		$data['access_token'] = isset($params['access_token']) ? $params['access_token'] : NULL; 

		return $data;
	} 
	else {
		return NULL;
	}
}