Facebook Webhook - Verification

facebook-webhook.php

<?php

class Hooks {
  /**
   * Facebook App Secret Key
   */
  protected $secretKey;

  /**
   * Webhook token which needed for Verification
   */
  protected $token; 

  public function __construct(string $secretKey, string$token)
  {
    $this->token = $token; 
    $this->secretKey  = $secretKey;
  }

 
  public function verify() {
    if ($_GET('hub_mode') === 'subscribe'
            && $_GET('hub_verify_token') === $this->token) {
            echo $_GET('hub_challenge');
        } else {
            echo 'invalid token';
        }
  }

  /**
   * Is valid x-hub signature
   */
  protected function isValidXHubSignature($data, $header_signature) 
  {
    $expected_signature = hash_hmac('sha1', $data, $this->secretKey);

    $signature = '';
    if (strlen($header_signature) == 45 && substr($header_signature, 0, 5) == 'sha1=') {
        $signature = substr($header_signature, 5);
    }

    return hash_equals($signature, $expected_signature); 
  }

  public function process() 
  {
    $data    = file_get_contents('php://input');
    $header_signature = $_SERVER('HTTP_X-Hub-Signature');

    if ($this->isValidXHubSignature($data, $header_signature)) {
      // Process your data
    } else {
      // return a 401 record.
    }
  }
}

$token = ''; 
$secretKey = '';

$hooks = new Hooks($secretKey, $token);

// Call this _GET
$hooks->verify();

// call this _POST
$hooks->process() ;