<?php
    $mongo = new MongoClient();
    $db = $mongo->dbName;
    $collection = $db->collectionName;

    $cursor = $collection->find(
        $criteria, 
        array('_id' => 1)
    )->limit(1);

    if($coursor->count() == 0){
      exit('not exist');
    }

    exit('exits');