setting featured image from first image in post

Hey Jonathon Parris,

Thank you for the inquiry.

You can try the following code in the functions.php file. It should work for both the actual URL and the image element.

function av_auto_set_featured_image($post_id) {
    if (wp_is_post_autosave($post_id) || wp_is_post_revision($post_id)) {
        return;
    }

    if (has_post_thumbnail($post_id)) {
        return;
    }

    $post = get_post($post_id);
    $content = $post->post_content;

    $image_url = '';

    if (preg_match('/<img.*?src=["\'](https?:\/\/.*?\.(jpg|jpeg|png|gif))["\']/', $content, $matches) || preg_match('/(https?:\/\/.*?\.(jpg|jpeg|png|gif))/', $content, $matches)) {
        $image_url = $matches[1];
    }

    if ($image_url) {
        $attachment_id = insert_attachment_from_url($image_url, $post_id);
        if ($attachment_id) {
            set_post_thumbnail($post_id, $attachment_id);
        }
    }
}
add_action('save_post', 'auto_set_featured_image');

function av_insert_attachment_from_url($url, $post_id) {
    $temp_file = download_url($url);
    if (is_wp_error($temp_file)) {
        return false;
    }

    $file = array(
        'name'     => basename($url),
        'type'     => mime_content_type($temp_file),
        'tmp_name' => $temp_file,
        'error'    => 0,
        'size'     => filesize($temp_file),
    );

    $sideload = wp_handle_sideload($file, array('test_form' => false));
    if (!empty($sideload['error'])) {
        @unlink($temp_file);
        return false;
    }

    $attachment = array(
        'post_mime_type' => $sideload['type'],
        'post_title'     => sanitize_file_name($file['name']),
        'post_content'   => '',
        'post_status'    => 'inherit',
    );

    $attachment_id = wp_insert_attachment($attachment, $sideload['file'], $post_id);

    require_once(ABSPATH . 'wp-admin/includes/image.php');
    $attach_data = wp_generate_attachment_metadata($attachment_id, $sideload['file']);
    wp_update_attachment_metadata($attachment_id, $attach_data);

    return $attachment_id;
}

Best regards,
Ismael