This snippet of php eZ Publish 5 (Symfony Stack) code uses the legacy kernel api to login to the user you wish to use to perform actions within eZ Publish Legacy as! Default admin user used in this example. Reference: http://share.ez.no/forums/ez-publish-5-platform/interracting-with-legacy-from-symfony-command#comment84674

symfony-legacy-kernel-callback-user-login-example-snippet.php

<?php

protected function execute(InputInterface $input, OutputInterface $output) { 
    $repository = $this->getContainer()->get('ezpublish.api.repository');
    $legacyKernel = $this->getContainer()->get('ezpublish_legacy.kernel');
    $userID = 14; // UserID of the Default Admin User
    $user = $repository->getUserService()->loadUser( $userID );
    $repository->setCurrentUser( $user );
    $result = $legacyKernel()->runCallback(
        function () use( $userID ) 
        {
            /** Login code to run as admin user. This is required to 
                see past content tree permissions, sections and other limitations **/
            $currentuser = eZUser::currentUser();
            $currentuser->logoutCurrent();
            $user = eZUser::fetch( $userID ); 
            $user->loginCurrent();
 
            return  \eZUser::currentUserID();
        }
    );
    $output->writeln( $result );
?>