Collection Item (Page) URL as a Variable for Share Buttons

Hi, I have set up my url as a variable to pass into the attributes and it works great for my collection items (I can see it in the source for the meta data). The following code lives in my header.php file:

    <?php 
        $domain        = 'https://'.$_SERVER["HTTP_HOST"];
        $url           = $domain.$_SERVER["REQUEST_URI"];
        PerchSystem::set_var('domain',$domain);
        PerchSystem::set_var('url',$url);
        ?>

What I want to do now is use that variable value for my share button links in the collection item. I add the following into the template file but it doesn't show the url for the collection item's page:

<perch:content id="url" type="hidden" />

Here is the full code for displaying the collection item and getting content into the meta data (post.php):

<?php
# get the post
$post = perch_collection('Blog', [
      'filter'        => 'titleSlug',
      'match'         => 'eq',
      'value'         => perch_get('s'),
      'skip-template' => 'true',
      'return-html'   => 'true',
    ]);

# set up the variables
$title       = $post['0']['heading'];
$description = $post['0']['desc'];
$keywords = $post['0']['keywords'];
$noindex = $post['0']['noindex'];
$nofollow = $post['0']['nofollow'];
$nosnippet = $post['0']['nosnippet'];
if (isset($post[0]['postImageSocial'])) {
    $socialImage = $post[0]['postImageSocial'];
} else {
    $socialImage = '';
}

if (isset($post[0]['author']) && is_array($post[0]['author'])) {
    foreach($post[0]['author'] as $authorID) {
        $author = perch_collection('Authors', [
             '_id'=>$authorID,
             'skip-template' => true,
        ]);
        // do whatever you will with the $author.
        $authorfirst = $author['0']['author_first_name'];
        $authorlast = $author['0']['author_last_name'];
    }
}


# use the variables in the array value 
perch_page_attributes_extend(array(
    'description'    => $description,
    'keywords'       => $keywords,
    'noindex'        => $noindex,
    'nofollow'       => $nofollow,
    'nosnippet'      => $nosnippet,
    'sitename'       => 'CPI',
    'og_description' => $description,
    'og_title'       => $title,
    'og_author'      => $authorfirst . ' ' . $authorlast,
    'og_type'        => 'article',
    'og_image'       => $socialImage,
    'sharing_image'  => $socialImage,
    'url'            => $url,
));
?>


<?php
  // Include the header. You can find this in tempates/layouts/global
  perch_layout('global/header', [
    'title' => $title,
    'body-class' => 'blog-post',
  ]);
?>


<?php 
# display the blog post
  echo $post['html']; 
?>


<?php
    // Include the footer. You can find this in tempates/layouts/global
    perch_layout('global/footer');
?>