WordPress - Automatically generate meta description from content

auto-description-tag-wp

function create_meta_desc() {
    global $post;
if (!is_single()) { return; }
    $meta = strip_tags($post->post_content);
    $meta = strip_shortcodes($post->post_content);
    $meta = str_replace(array("\n", "\r", "\t"), ' ', $meta);
    $meta = substr($meta, 0, 125);
    echo "<meta name='description' content='$meta' />";
}
add_action('wp_head', 'create_meta_desc');

Thanks for this gist, but it need little update for people who migrate from classic editor to Gutenberg.

You must add"<!-- wp:paragraph -->", "<p>"to str_replace, in Gutenberg all paragraphs are begining (and ending) with <!-- wp:paragraph --> also sometimes Gutenberg add <p> to html.

// $meta = str_replace( array("\n", "\r", "\t", "<!-- wp:paragraph -->", "<p>", "<!--nextpage-->"), ' ', $meta );