Is there a way to test MongoDB connection in PHP?

Ok, I found a way to test it!

<?php
$mongo = new MongoDB\Client('mongodb://my_server_does_not_exist_here:27017');
$dbs = $mongo->listDatabases();

If the connection fails, listDatabases throws a MongoDB\Driver\Exception\ConnectionTimeoutException. You just need to set a try/catch around the listDatabases.