我有一个问题是在没有插件的情况下在Wordpress中进行3级自定义大型菜单。现在它是静态HTML，但我想在PHP中动态地使用它。我的header.php代码如下所示：
<!-- START offcanvas -->
<div class="offcanvas visible-xs-block visible-sm-block">
<a href="#" class="mobile-nav-toggle"></a>
<nav>
<li><a href="#dropdown-angebote" data-toggle="collapse" class="has-dropdown">Unser Angebot</a></li>
<li><a href="/das-magazin">Das Magazin</a></li>
<li><a href="/reservisten-vor-ort">Reservisten vor Ort</a></li>
<li><a href="/der-verband">Der Verband</a></li>
<li><a href="#">Mitgliedschaft</a></li>
</nav>
<nav class="visible-xs-block">
<?php wp_header_meta_nav(); ?>
<li>
<a href="#" class="icon-search">
<svg width="18" height="18">
<image xlink:href="<?php echo get_template_directory_uri(); ?>/dist/img/icon-search.svg" src="<?php echo get_template_directory_uri(); ?>/dist/img/icon-search.png" width="18" height="18"/>
</svg>
<?php _e( 'Suche', 'reservistenverband' ) ?>
</a>
</li>
<li><a href="#" class="btn btn--yellow text-uppercase"><?php _e( 'Mitglied werden', 'reservistenverband' ) ?></a></li>
<li><a href="#" class="text-uppercase"><?php _e( 'Login', 'reservistenverband' ) ?></a></li>
</nav>
</div>
<!-- END offcanvas -->
<!-- START Header-Dropdowns -->
<div class="header-dropdowns">
<div class="header-dropdown collapse" id="dropdown-angebote">
<a href="#" class="dropdown-close visible-xs-block visible-sm-block"></a>
<div class="container">
<div class="row">
<div class="col-md-4 col-left">
<ul>
<li><a href="/veranstaltungen">Veranstaltungen</a></li>
<li><a data-toggle="collapse" href="#dropdown-truppe" class="has-dropdown">Reservisten in der Truppe</a></li>
<li><a href="#">Internationale Arbeit</a></li>
<li><a data-toggle="collapse" href="#dropdown-ausbildung" class="has-dropdown">Militärische Aus-und Weiterbildung</a></li>
<li><a href="#">Öffentlichkeitsarbeit</a></li>
<li><a data-toggle="collapse" href="#dropdown-sicherheitspolitisch" class="has-dropdown">Sicherheitspolitische Arbeit</a></li>
<li><a href="#">Verbandsarbeit</a></li>
</ul>
</div>
<div class="col-md-4 col-right">
<div class="header-sub-dropdown collapse" id="dropdown-ausbildung">
<a href="#" class="dropdown-close visible-xs-block visible-sm-block"></a>
<ul>
<li><a href="/unser-angebot/militaerische-aus-und-weiterbildung/taktik">Taktik</a></li>
<li><a href="/unser-angebot/militaerische-aus-und-weiterbildung/fernmeldeausbildung">Fernmeldeausbildung</a></li>
<li><a href="/unser-angebot/militaerische-aus-und-weiterbildung/schiessen">Schießen</a></li>
<li><a href="/unser-angebot/militaerische-aus-und-weiterbildung/agshp">AGSHP</a></li>
<li><a href="/unser-angebot/militaerische-aus-und-weiterbildung/sanitaerausbildung">Sanitärausbildung</a></li>
<li><a href="/unser-angebot/militaerische-aus-und-weiterbildung/gefechtdienst">Gefechtsdienst</a></li>
</ul>
</div>
</div>
<div class="col-md-4 col-teaser">
<div class="tt tt--black">
<div class="tt__content">
<span class="tt__title text-uppercase">Unser Angebot</span>
<p class="tt__text">Pro Hauptnavigationspunkt kann hier<br/> ein besonderes Thema hervorgehoben und angeteasert werden.</p>
<a href="#" class="btn btn--dark-blue btn--big text-uppercase tt__btn--bottom">Mehr dazu</a>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
<!-- END Header-Dropdowns -->
<!-- START Search -->
<div class="fullscreen-search search">
<div class="search__overlay"></div>
<div class="search__form">
<div class="container">
<a href="#" class="search__close"></a>
<!--<h3 class="text-uppercase">Suche</h3>-->
<form action="">
<input type="text" class="search__input" placeholder="Schlagwort hier eingeben..."/>
</form>
</div>
</div>
</div>
<!-- END Search -->
<div class="page-wrapper">
<!-- START main-header -->
<header class="main-header">
<div class="pre-nav hidden-xs">
<div class="container">
<nav>
<?php wp_header_meta_nav(); ?>
<li>
<button class="icon-search">
<svg width="18" height="18">
<image xlink:href="<?php echo get_template_directory_uri(); ?>/dist/img/icon-search.svg" src="<?php echo get_template_directory_uri(); ?>/dist/img/icon-search.png" width="18" height="18"/>
</svg>
</button>
</li>
<li><a href="#" class="btn btn--yellow text-uppercase"><?php _e( 'Mitglied werden', 'reservistenverband' ) ?></a></li>
<li><a href="#" class="text-uppercase"><?php _e( 'Login', 'reservistenverband' ) ?></a></li>
</nav>
</div>
</div>
<div class="main-nav">
<div class="container">
<a href="#" class="mobile-nav-toggle visible-xs-block visible-sm-block">
<span></span>
<span></span>
<span></span>
</a>
<div class="row">
<div class="col-md-3">
<a href="<?php bloginfo( 'url' ); ?>">
<object type="image/svg+xml" data="<?php echo get_template_directory_uri(); ?>/dist/img/logo.svg">
<img src="<?php echo get_template_directory_uri(); ?>/dist/img/logo.png" alt="No SVG support">
</object>
</a>
</div>
<div class="col-md-9">
<nav class="hidden-sm hidden-xs">
<li><a href="#dropdown-angebote" data-toggle="collapse" class="text-uppercase has-dropdown">Unser Angebot</a></li>
<li><a href="#" class="text-uppercase">Das Magazin</a></li>
<li><a href="#" class="text-uppercase">Reservisten vor Ort</a></li>
<li><a href="#" class="text-uppercase">Der Verband</a></li>
<li><a href="#" class="text-uppercase">Mitgliedschaft</a></li>
<?php //wp_header_main_nav() ?>
</nav>
</div>
</div>
</div>
</div>
</header>
</div>
<!-- END main-header -->
我想让WP-Backend中的菜单使用PHP。我不知道如何使用PHP编写自定义WP导航助手或其他代码。谢谢你的任何建议。