function to add Site Navigation schema to wordpress site footer. >> https://schema.org/SiteNavigationElement

wp-navbar-schema-function.php

<?php
###########################################################
# Sabry Navbar Schema function
###########################################################

if (!function_exists('sabryschemanav')) {
  function sabryschemanav(){
    $menuLocations = get_nav_menu_locations();
    $menuID = $menuLocations['header-menu'];
    $primaryNav = wp_get_nav_menu_items($menuID);
    echo '<script type="application/ld+json">
    {
  	"@context": "https://schema.org",
  	"@graph":
  	[';
    $key4nav = 1;
    foreach ( $primaryNav as $navItem ) {
      if($key4nav != 1){echo',';}
      echo '{
          "@context": "https://schema.org",
          "@type":"SiteNavigationElement",
          "@id":"site-navigation-'.$key4nav.'",
          "name": "'.$navItem->title.'",
          "url": "'.replaculrslashes($navItem->url).'"
      }';
      $key4nav++;
    }
    echo '] } </script>';
    }
}

function addsabryschemanav() {
    sabryschemanav();
}
add_action( 'wp_footer', 'addsabryschemanav' );

function replaculrslashes($sjdkln) {
  $kkkurl = str_replace("/","\/",$sjdkln);
  return $kkkurl;
}