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<?php 
// Usage:
// get_id_by_slug('page-slug');

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

// WPML: hard-coding the link of a page
// get_permalink( get_id_by_slug( 'page-slug' ));