<?php
$usuario = 'root';
$password = '';
$db = new PDO('mysql:host=localhost;dbname=php_llenarselect', $usuario, $password);
?>

<?php
$query = $db->prepare("SELECT * FROM paises");
$query->execute();
$data = $query->fetchAll();

foreach ($data as $valores):
echo '<option value="'.$valores["id"].'">'.$valores["paises"].'</option>';
endforeach;
?>