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//* Get page ID from title...
function get_id_by_title( $page_title ) {
	
    $page = get_page_by_title( $page_title );
	
    if ( $page ) {
		
        return $page->ID;
    } 
    else {
		
        return null;
    }
}

//* Example...
$contact_id = get_id_by_title( 'Contact Us' );

//* Get page ID from slug...
function get_id_by_slug( $page_slug ) {
	
    $page = get_page_by_path( $page_slug );
	
    if ( $page ) {
		
        return $page->ID;
    } 
    else {
		
        return null;
    }
}

//* Example...
$about_id = get_id_by_slug( 'about' );