Wordpress get page ID by slug

extras.php

/**
* Get a page ID by slug
 * @param $page_slug
 *
 * @return int|null
 */
function bb_get_page_id_by_slug($page_slug) {
    $page = get_page_by_path($page_slug);
    if ($page) {
        return $page->ID;
    } else {
        return null;
    }
}