public function beforeFilter(Event $event) {
    if( true ){ //if a sample condition is true
        return $this->redirect([
            'controller' => 'test',
            'action' => 'action'
        ]);
    }

public function beforeFilter(Event $event) {
if( true ){ //if a sample condition is true
    $url = '/'; //set url here
    echo '<script type="text/javascript">';
        echo 'window.location="'.$url.'"';
    echo '</script>';
    exit;
}