insert_tag_to_html.php

#!/usr/bin/php
<?php
$olddom = new DOMDocument('1.0', 'utf-8');
$olddom->loadHTMLFile('arc.html');

$boby = $olddom->getElementsByTagName('body')->item(0);

$boby->insertBefore(
    $olddom->createElement('h1', 'Zippy'),
    $olddom->getElementsByTagName('p')->item(0)
);

foreach ($boby->childNodes as $key => $value) {
    $boby->insertBefore(
        $olddom->createElement('h1', 'Zippy'),
        $value
    );
}

echo $olddom->saveHTML();