Foundation Magellan auto generate - php

Question

I'd like to auto generate magellan on subpages from h3 tags.
For this purpose I tried jameswlane script (https://gist.github.com/jameswlane/f1eabeedf430ef67318d), and I changed only h2 tags to h3. It seems I dont aunderstand this code well.
I thought I only need to paste this code in my header.php file from wordpress theme and it will automatically sweep the DOM content for h3 tags, but it didn't.
Nothing shows up, even an empty div.
Used the followinf line as init:
<?php $tag = 'h3'; $html = get_the_content(); $items = getTextBetweenTags( $tag, $html ); ?>
Can I ask you for an advise in this matter?
<?php
function getTextBetweenTags($tag, $html, $strict=0) {
$dom = new domDocument;
if($strict==1) {
$dom->loadXML($html);
} else {
$dom->loadHTML($html);
}
$dom->preserveWhiteSpace = false;
$content = $dom->getElementsByTagname($tag);
$out = array();
foreach ($content as $item) {
$out[] = $item->nodeValue;
}
return $out;
}
function create_slug($string){
$slug=preg_replace('/[^A-Za-z0-9-]+/', '-', $string);
return $slug;
}
function create_magellan($items){
$output .= '<div data-magellan-expedition="fixed">' . PHP_EOL;
$output .= '<dl class="sub-nav">' . PHP_EOL;
foreach( $items as $item ) {
$output .= '<dd data-magellan-arrival="' . create_slug( $item ) . '">' . $item . '</dd>';
}
$output .= '</dl>' . PHP_EOL;
$output .= '</div>' . PHP_EOL;
add_filter('the_content', 'my_formatter', 99);
echo $output;
}
function my_formatter($content) {
$new_content = '';
$tag = 'h3';
$pattern_full = '{(<' . $tag . '>.*?</' . $tag . '>)}is';
$pattern_contents = '{<' . $tag . '>(.*?)</' . $tag . '>}is';
$pieces = preg_split($pattern_full, $content, -1, PREG_SPLIT_DELIM_CAPTURE);
foreach ($pieces as $piece) {
if (preg_match($pattern_contents, $piece, $matches)) {
$test = $matches[1];
$new_content .= '<a name="'.create_slug($test).'"></a>'.PHP_EOL.'<'.$tag.' data-magellan-destination="'.create_slug($test).'">'.$test.'</'.$tag.'>'.PHP_EOL;
} else {
$new_content .= wptexturize(wpautop($piece));
}
}
return $new_content;
}
?>
EDIT
Something worked and returned error:
Warning: DOMDocument::loadHTML(): Empty string supplied as input in /www/ilee_www/www/2.goldbaltic/dev.goldbaltic.pl/wp-content/themes/dermiclab_fs/partials/2.Nav/mainNav.php on line 47
the line number 47 in my file is
$dom->loadHTML($html);
from
function getTextBetweenTags($tag, $html, $strict=0) {
$dom = new domDocument;
if($strict==1) {
$dom->loadXML($html);
} else {
$dom->loadHTML($html);
}
$dom->preserveWhiteSpace = false;
$content = $dom->getElementsByTagname($tag);
$out = array();
foreach ($content as $item) {
$out[] = $item->nodeValue;
}
return $out;
}