如何从php连接到mongodb [openshift]？

Im trying to connect to mongodb. Use openshift hosting. Code u can see in footer. I tried to enter username,pass before $env and db_name after $env, but nothing happens. I get 500 error from webserver. Can somebody tell me whats wrong and how to fix it? rockmongo works fine, so mistake in my code, i think. i tried lot of combination of it

<?php

// connect to MongoDB
$m = new MongoClient("mongodb://$OPENSHIFT_MONGODB_DB_HOST:$OPENSHIFT_MONGODB_DB_PORT");

// select a database
$db = $m->comedy;

// select a collection 
$collection = $db->cartoons;

$document = array( 
      "title" => "Hibernate and MongoDB", 
      "author" => "Anghel Leonard"
   );

//insert a new document
  $collection->insert($document);

//returns a cursor for the search results
$cursor = $collection->find();

// iterate through the results
foreach ($cursor as $document) {
    echo '"_id": '.$document["_id"]."<br />";
    echo '"title": '.$document["title"]."<br />";
    echo '"author": '.$document["author"]."<br />";
    echo '*********************************';
} 
?>

u need to use "OPENSHIFT_MONGODB_DB_URL", not $OPENSHIFT_MONGODB_DB_HOST:$OPENSHIFT_MONGODB_DB_PORT that u get after mongodb creation