WP Block Theme div to main

<?php

function replace_content_div_with_main() {
    ob_start(function($buffer) {
        $dom = new DOMDocument();
        $dom->loadHTML(mb_convert_encoding($buffer, 'HTML-ENTITIES', 'UTF-8'), LIBXML_HTML_NOIMPLIED | LIBXML_HTML_NODEFDTD);

        $xpath = new DOMXPath($dom);
        $divs = $xpath->query('//div[contains(@class, "wp-site-blocks")]');

        if ($divs->length > 0) {
            foreach ($divs as $div) {
                $containsHeader = $xpath->query('.//header', $div)->length > 0;
                $containsFooter = $xpath->query('.//footer', $div)->length > 0;

                if (!$containsHeader && !$containsFooter) {
                    $main = $dom->createElement('main');
                    $main->setAttribute('class', $div->getAttribute('class'));

                    while ($div->firstChild) {
                        $main->appendChild($div->firstChild);
                    }

                    $div->parentNode->replaceChild($main, $div);
                    break;
                }
            }
        }
        $buffer = $dom->saveHTML();
        return $buffer;
    });
}
add_action('wp_loaded', 'replace_content_div_with_main');

function end_output_buffer() {
    if (ob_get_length()) {
        ob_end_flush();
    }
}
add_action('shutdown', 'end_output_buffer', 999);