PHP-PDO: Obtendo o último ID do insert

CREATE TABLE IF NOT EXISTS `nome_tabela` (
  `id` int(5) unsigned zerofill AUTO_INCREMENT NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

CREATE TABLE IF NOT EXISTS `nome_tabela` ( `id` int(5) unsigned zerofill AUTO_INCREMENT NOT NULL, PRIMARY KEY (`id`) ) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

<? 
   require_once("conexao.php"); 
   $pdo->exec("INSERT INTO nome_tabela VALUES()"); 
   $last_id = $pdo->lastInsertId();
   echo $last_id;
?>

<? require_once("conexao.php"); $pdo->exec("INSERT INTO nome_tabela VALUES()"); $last_id = $pdo->lastInsertId(); echo $last_id; ?>