Solution to doing some processing for every method of FuelPHP REST controller. Also add limit supported data type.

gistfile1.php

<?php

class ExampleRestController extends Controller_Rest {

  /**
   * @var  array  List all supported methods
   */
  protected $_supported_formats = array(
    'xml' => 'application/xml',
  //  'rawxml' => 'application/xml',
    'json' => 'application/json',
  //  'jsonp'=> 'text/javascript',
  //  'serialized' => 'application/vnd.php.serialized',
  //  'php' => 'text/plain',
  //  'html' => 'text/html',
  //  'csv' => 'application/csv',
  );

  /**
   * Make sure the router method has the same method signature as the parent::router declaration.
   * @see fuel/core/classes/controller/rest.php
   */
  public function router($method, array $params) {

    $defaultResponse = array('resp'=>'NG', 'status'=>'authentication key doesn\'t recognized.');
    if(!static::validateMyAPIKey()){

      if($this->format == "json"){
      $response = Response::forge(Format::forge( $defaultResponse )->to_json())
                  ->set_header('Content-Type', $this->_supported_formats[$this->format]);
      } else if($this->format == $xml) {
      $response = Response::forge(Format::forge( $defaultResponse )->to_xml())
                  ->set_header('Content-Type', $this->_supported_formats[$this->format]);
      }
      return $response;
    }

}