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jQuery.ajax({
  dataType: 'json',
  url: '/users',
  data: {
    id: 12345678
  },

  // If HTTP response status code is 4XX (error)
  // Check status codes for more info: http://httpstatus.es
  error: function(xhr) {
    /** @type {number} HTTP response status code */
    xhr.status;

    /** @type {string} HTTP response text */
    xhr.responseText;
  },

  // If HTTP response status code is 2XX (success)
  success: function(data, status, xhr) {
    /** @type {Object} HTTP response body */
    data;

    /** @type {number} HTTP response status code */
    xhr.status;
  }
});

<?php

class Users_Controller extends Base_Controller {

  public function action_index() {
    $id = Input::get('id');

    if ($id) {
      $user = DB::table('users')->where('id', $id)->first();
      
      if (count($user)) {
        // If user (queried by id) has been found in the database,
        // returning JSON with it's data, with 200 HTTP request status.
        return Response::json($user, 200);
      }
    }
    
    // Otherwise, if user id is not provided or user is not found in the
    // database, returning error message with 404 HTTP request status.
    return Response::make('User not found', 404);
  }