php类未找到致命错误

We have implemented Facebook login with php 5.6,Class not found fatal error occurs.

Now,I am running it in the local development environment, but I got a question as I got an error.

Please tell me why class is not found.

*fb_sighup.php

require_once($_SERVER['DOCUMENT_ROOT'] . '/config/fb_login_config.php');

$fblogin = new MyApp\Facebooklogin(); //error occured

try {
  $fblogin->sighup();
} catch (Exception $e) {
  echo $e->getMessage();
  exit;
}
?>

*Facebooklogin class

 class Facebooklogin {
  private $_fb;

  public function __construct() {
    $this->_fb = new \Facebook\Facebook([
      'app_id' => APP_ID,
      'app_secret' => APP_SECRET,
      'default_graph_version' => APP_VERSION,
    ]);
  }

  public function sighup() {
    $helper = $this->_fb->getRedirectLoginHelper();

    $loginUrl = $helper->getLoginUrl(CALLBACK_URL);
    header('Location: ' . $loginUrl);
    exit;
  }
}//class facebooklogin
?>

In addition, FacebookSDK is installed via composer.json

The problem is to do with namespacing. Assuming the code for your Facebooklogin class is complete.

You have 2 options here. You can remove the namespacing for the instigation of the Facebooklogin class by changing

$fblogin = new MyApp\Facebooklogin();

$fblogin = new Facebooklogin();

or you can add the namespacing in

namespace MyApp {
    class Facebooklogin {
        ....
    }
}

And in the calling code

use MyApp;

$fblogin = new MyApp\Facebooklogin();

I find this Stackoverflow question and answer may help Cannot find Class with PHP Namespace

$_SERVER['DOCUMENT_ROOT']

dirname(____FILENAME__)