Fatal error: Class 'Phalcon\Mvc\Application' not found in /var/www/public/index.php on line 6

<?php

error_reporting(E_ALL); ini_set("display_errors","on"); class Application extends \Phalcon\Mvc\Application {

/**
 * Register the services here to make them general or register in the ModuleDefinition to make them module-specific
 */
protected function _registerServices()
{

    $di = new \Phalcon\DI\FactoryDefault();

    //Setup the view component
    $di->set('view', function(){
        $view = new \Phalcon\Mvc\View();
        $view->setViewsDir('../app/views/');
        $view->registerEngines(array(
                ".volt" => 'Phalcon\Mvc\View\Engine\Volt'
        ));
        return $view;
    });

    $loader = new \Phalcon\Loader();

    /**
     * We're a registering a set of directories taken from the configuration file
     */
    $loader->registerDirs(
        array(
            __DIR__ . '/../apps/library/'
        )
    )->register();

    //Registering a router
    $di->set('router', function(){

        $router = new \Phalcon\Mvc\Router();

        $router->setDefaultModule("frontend");

        $router->add('/:controller/:action', array(
            'module' => 'frontend',
            'controller' => 1,
            'action' => 2,
        ));

        $router->add("/admin/:controller/:action", array(
            'module' => 'backend',
            'controller' => 1,
            'action' => 2,
        ));

        return $router;

    });

    $this->setDI($di);
}

public function main()
{

    $this->_registerServices();

    //Register the installed modules
    $this->registerModules(array(
        'frontend' => array(
            'className' => 'Multiple\Frontend\Module',
            'path' => '../apps/frontend/Module.php'
        ),
        'backend' => array(
            'className' => 'Multiple\Backend\Module',
            'path' => '../apps/backend/Module.php'
        )
    ));

    echo $this->handle()->getContent();
}

}

$application = new Application(); $application->main();