Parse error with Vietnamese characters or open and close tag only about shortcode HOT 6 CLOSED

<?php
require_once 'vendor/autoload.php';

use Thunder\Shortcode\HandlerContainer\HandlerContainer;
use Thunder\Shortcode\Parser\RegularParser;
use Thunder\Shortcode\Processor\Processor;
use Thunder\Shortcode\Shortcode\ShortcodeInterface;
use Thunder\Shortcode\Syntax\Syntax;

function shortcode($text) {
    $handlers = new HandlerContainer();
    $handlers->add('container', function(ShortcodeInterface $s) {
        return '<div class="container">'.$s->getContent().'</div>';
    });
    $defaultSyntax = new Syntax('[', ']', '/', '=', '"');
    $processor = new Processor(new RegularParser($defaultSyntax), $handlers);
    return $processor->process($text);
}

$case_1 = '[container][Tiêu đề][/container]';
$case_2 = '[container][][/container]';

echo shortcode($case_1); // I get <div class="container"></div>[Tiêu đề][/container]
echo shortcode($case_2); // I get <div class="container"></div>[][/container]