How to use injection class in php?

I have a class:

class MongoConnection
{

    private $_m;

    private $_db;

    private $_collection;

    public function _connect($dbname, $dbcollection)
    {
    }

}

In another class I want to use this like as:

class Object
{

    private $collection;


    public function __construct(MongoConnection $mongoConnection)
    {

       $mongoConnection->_connect("", "");
    }

}

But it does not work for me. I get an error:

Catchable fatal error: Argument 1 passed to Object::__construct() must be an instance of MongoConnection, none given, called in..

I create instance of Object class as: $object = new Object();