我有以下代码谁应该工作良好。但Wordpress正在崩溃。怎么了？

echo "<ul>";
$usernames = $wpdb->get_results("SELECT user_nicename, user_url FROM $wpdb->users ORDER BY ID DESC LIMIT 5");
foreach ($usernames as $username) {
    echo \'<li><a href="\'.$username->user_url.\'">\'.$username->user_nicename."</a></li>";
}
echo "</ul>";