Well most of the tutorials didn't explained well, So i hope this might help someone 
Note: this is a part of my laravel project  

//getting data from a collection
<?php

use MongoDB\Client as Mongo;

$user = "admin";
$pwd = 'password';

$mongo = new Mongo("mongodb://${user}:${pwd}@127.0.0.1:27017");
$collection = $mongo->db_name->collection;
$result = $collection->find()->toArray();

print_r($result);

?>

When your database name contains a "-" (e.g. database-name) you need to use a string instead.

<?php

$client = new MongoDB\Client("mongodb://ip_address:port");
$collection = $client->{'database-name'}->collection;

?>

To test your connection string, you can do something like this:

<?php
$mongo = new MongoDB\Client('mongodb://my_server_does_not_exist_here:27017');
try 
{
    $dbs = $mongo->listDatabases();
}
catch (MongoDB\Driver\Exception\ConnectionTimeoutException $e)
{
    // PHP cannot find a MongoDB server using the MongoDB connection string specified
    // do something here
}
?>

If you have a number of JSON documents with nested elements such as 'responseId' below and you want to know how many documents have a responseId:
{"result":{"responseId":"xyz"}}
{"result":NULL}
{"result":{"responseId":"abc"}}

I was not having luck with the following format
<?php 
// trying to get the count of documents where responseId is not equal to NULL (did not work for me)
$intCount = $collection->count(['result' => ['responseId' => ['$ne' => NULL]]]);
?>

Instead I needed to use a period between the JSON elements:
<?php
// get the count of documents where responseId is not equal to NULL
$intCount = $collection->count(['result.responseId' => ['$ne' => NULL]]);
?>

Do a text search on the collection with projection

$search['$text'] = ['$search' => "foo"];
$options["projection"] = ['score' => ['$meta' => "textScore"]];
$options["sort"] = ["score" => ['$meta' => "textScore"]];

$cursor = $collection->find($search, $options);

Pecl MongoDB at time of writing can be installed (see phpinfo()) but composer will complain that it's not present.

$  composer require "mongodb/mongodb=^1.0.0"
...
Your requirements could not be resolved to an installable set of packages.

If you see this try 

$ composer require "mongodb/mongodb=^1.0.0" --ignore-platform-reqs

One question that was unanswered was how to handle insertion of a full JSON document provided as an argument (or in a string variable). The mongodb extension can handle this by simply using json_decode() to convert the JSON document to an object that can then be easily inserted, e.g. (with full document provided as the 2nd command-line argument)

<?php
/* include libmongo API */
require "vendor/autoload.php";

/* include connection string, db and collection values */
require __DIR__ . '/site/db-mongo-inc.php';

/* use MongoDBClient; */
use MongoDB\Client;

/* connection string */
$uri = "mongodb://$user:$pass@localhost";

/* attempt connection to database with $uri */
$client = new MongoDB\Client("$uri");
if (!isset($client)) {
  echo 'error: connection failed';
}

/* define collection of documents */
$collection = $client->$db->$collection;

/* insert JSON document from string variable (here $argv[2]) */
if ($argc > 2) {
  $jsobj = json_decode ($argv[2]);
  if ($jsobj) {
    $iresult = $collection->insertOne ( $jsobj );
    if (!$iresult) {
      printf ("error: insert of object failed.\n");
    }
  }
}
?>

The extension's block insert feature recursively resolves all field and data values contained in the object and inserts the entire document in the collection. It is helpful to remove the "_id:" field and have that auto-generated for you to avoid issues.