Troubles when I want to connect to another DB

Use code as below :

<?php

$myotherdatabase = new wpdb("username","password","databasename","localhost");

$motscles = "abri";

$query = "SELECT `post_id` FROM `wp_postmeta` WHERE `meta_key` = 'nom' AND MATCH (`meta_value`) AGAINST ('" . $motscles . "' IN BOOLEAN MODE)";

$result = $myotherdatabase -> get_results($query, ARRAY_A);

foreach ($result as $post) {
  $pids .= $post->post_id . ",";// ARRAY_A
}

echo $pids;

?>