wp-get_id_by_slug

// Usage:
// get_id_by_slug( 'any-page-slug' );

function get_id_by_slug( $page_slug ) {
	$page = get_page_by_path( $page_slug );
	
	if ( $page ) {
		return $page->ID;
	} else {
		return null;
	}
}