Adding breadcrumb and side menus to WP_Themes

page.php

<!-- breadcrumb -->
    <?php
    $theParent = wp_get_post_parent_id(get_the_ID());
    if ($theParent) { ?>
        <div class="metabox metabox--position-up metabox--with-home-link">
      <p><a class="metabox__blog-home-link" href="<?php echo get_permalink($theParent); ?>"><i class="fa fa-home" aria-hidden="true"></i> Back to <?php echo get_the_title($theParent); ?></a> <span class="metabox__main"><?php the_title(); ?></span></p>
    </div>
      <?php }
    ?>


<!-- side_menu Only appears if the page has child pages -->
<?php 

$testArray = get_pages(array(
    'child_of' => get_the_ID()
));

if($theParent or $testArray) { ?>
    
    <div class="page-links">
      <h2 class="page-links__title"><a href="<?php echo get_permalink($theParent); ?>"><?php  echo get_the_title($theParent); ?> </a></h2>
      <ul class="min-list">
        <?php

        if($theParent){
            $findChildrenOf = $theParent;
        }else{
            $findChildrenOf = get_the_ID();
        }
            wp_list_pages(array(
                'title_li' => NULL,
                'child_of' => $findChildrenOf,
                'sort_column' => 'menu_order'

            ));
        ?>

      </ul>
    </div>
<?php } ?>