add_action('init', function() {
    if (!isset($_GET['delete_posts'])) return;

    global $wpdb;

    $batch = 5000;

    $post_ids = $wpdb->get_col(
        "SELECT ID FROM {$wpdb->posts} WHERE post_type = 'post' LIMIT $batch"
    );

    if (empty($post_ids)) {
        echo "Done.";
        exit;
    }

    $ids_str = implode(',', array_map('intval', $post_ids));

    $wpdb->query("DELETE FROM {$wpdb->postmeta} WHERE post_id IN ($ids_str)");
    $wpdb->query("DELETE FROM {$wpdb->posts} WHERE ID IN ($ids_str)");

    echo count($post_ids) . " posts deleted.";
    exit;
});
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home/standa9/public_html/wp-content/mu-plugins/index.php:1) in <b>/home/standa9/public_html/wp-content/plugins/seo-by-rank-math/includes/modules/sitemap/abstract-xml.php</b> on line <b>66</b><br />
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home/standa9/public_html/wp-content/mu-plugins/index.php:1) in <b>/home/standa9/public_html/wp-content/plugins/seo-by-rank-math/includes/modules/sitemap/abstract-xml.php</b> on line <b>69</b><br />
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home/standa9/public_html/wp-content/mu-plugins/index.php:1) in <b>/home/standa9/public_html/wp-content/plugins/seo-by-rank-math/includes/modules/sitemap/abstract-xml.php</b> on line <b>69</b><br />
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home/standa9/public_html/wp-content/mu-plugins/index.php:1) in <b>/home/standa9/public_html/wp-content/plugins/seo-by-rank-math/includes/modules/sitemap/abstract-xml.php</b> on line <b>69</b><br />
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home/standa9/public_html/wp-content/mu-plugins/index.php:1) in <b>/home/standa9/public_html/wp-content/plugins/seo-by-rank-math/includes/modules/sitemap/abstract-xml.php</b> on line <b>69</b><br />
<br />
<b>Warning</b>:  Cannot modify header information - headers already sent by (output started at /home/standa9/public_html/wp-content/mu-plugins/index.php:1) in <b>/home/standa9/public_html/wp-content/plugins/seo-by-rank-math/includes/modules/sitemap/abstract-xml.php</b> on line <b>69</b><br />
<?xml version="1.0" encoding="UTF-8"?><?xml-stylesheet type="text/xsl" href="//solar2.standardof.net/main-sitemap.xsl"?>
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
	<sitemap>
		<loc>https://solar2.standardof.net/post-sitemap.xml</loc>
		<lastmod>2020-11-11T12:01:08+00:00</lastmod>
	</sitemap>
	<sitemap>
		<loc>https://solar2.standardof.net/page-sitemap.xml</loc>
		<lastmod>2020-11-12T00:11:20+00:00</lastmod>
	</sitemap>
	<sitemap>
		<loc>https://solar2.standardof.net/category-sitemap.xml</loc>
		<lastmod>2020-11-11T12:01:08+00:00</lastmod>
	</sitemap>
</sitemapindex>
<!-- XML Sitemap generated by Rank Math SEO Plugin (c) Rank Math - rankmath.com -->