How to split Contao variable into several HTML Elements - php

Question

I´ve got this template from my Contao CMS:
<?php $helper = new \ContaoBootstrap\Navbar\Helper\NavigationHelper($this); ?>
<?php foreach ($this->items as $item) : ?>
<?php
$currentLevelStringParts = explode('_', $this->level);
$currentLevelNumber = $currentLevelStringParts[1];
?>
<?php $itemHelper = $helper->getItemHelper($item); ?>
<li class="<?php echo str_replace(array('_'), array('-'), $itemHelper->getItemClass());
if (!empty($item['subitems'])) {echo ' subnav';} ?>
">
<<?= str_replace(array('strong'), array('a'), $itemHelper->getTag()); ?>
<?= str_replace(array('dropdown-item', 'dropdown-toggle', 'nav-link', 'data-toggle="dropdown"'), array(' ', ' ', ' ', ' '), $itemHelper) ?>>
<?php if ($item['isActive']) : ?>
<?= $item['link']?>
<?php else : ?>
<span itemprop="name"><?= $item['link']?></span>
<?php endif; ?>
</<?= str_replace(array('strong'), array('a'), $itemHelper->getTag()); ?>>
<?php if (!empty($item['subitems'])) { ?>
<span class="subnav-pull-down hidden-lg-up <?php if ($item['isTrail']) : ?>active<?php endif; ?>"></span>
<div class="subnav-container <?php if ($item['isTrail']) : ?>open<?php endif; ?>">
<div class="relative">
<ul class="nav sub-nav level-<?php echo (int)$currentLevelNumber+1 ?>">
<?= $item['subitems'] ?>
<div class="clear"></div>
</ul>
</div>
</div>
<?php } ?>
</li>
<?php endforeach; ?>
My problem is, I need a HTML structure like:
<ul>
<li><a></a></li>
<li><a></a></li>
<li>
<ul>
<li><a></a></li>
</ul>
</ul>
To change link () and description for my navigation. I tried to work the the bootstrap API but I´m kind of new to PHP and Bootstrap.
I hope you guys can help me.