When executing following PHP code:

$m = new MongoClient("mongodb://localhost:27017");

I get following error:

Fatal error: Class 'MongoClient' not found in (...)

MongoDB extension seems properly installed (I copied php_mongodb.dll to ext folder and updated php.ini).

PHP seems to confirm that extension is running properly as the following code confirms it is loaded:

echo extension_loaded("mongodb") ? "loaded\n" : "not loaded\n";

Also, phpinfo() shows that mongodb extension has been loaded.

UPDATE

Problem still not solved.

phpinfo() clearly shows that the driver is loaded:

But I still receive the same fatal error.

TL; DR

The class MongoClient is part of the legacy PECL package mongo but not anymore of the up-to-date mongodb package.

On MongoDB PHP driver github repo, the release note about the version 1.0.0, is suggesting developers to use MongoDB\Driver\Manager instead of MongoClient

Changes from our legacy mongo extension

Most significantly, the legacy driver's MongoClient, MongoDB, and MongoCollection classes have been obsoleted by the MongoDB\Driver\Manager class, which is the new gateway for connecting and executing queries, commands, and write operations.

Source: https://github.com/mongodb/mongo-php-driver/releases/tag/1.0.0

As the documentation is prompting it, the class is deprecated.

Warning This extension that defines this class is deprecated. Instead, the MongoDB extension should be used. Alternatives to this class include:

MongoDB\Driver\Manager

Source: http://php.net/MongoClient

From what I read on their github repository release history, the class you are trying to use have been obsoleted since the version of mongodb 1.0.0, so, on the version 1.6.0 you are, this class is not even part of the dll anymore.

That is confirmed by this issue on their github

derickr commented on Apr 16

MongoClient is a class from the old legacy driver and is not supposed to be available in this one. The new driver has \MongoDB\Driver\Manager, and, the accompanying library has \MongoDB\Client.

You either need to install the old legacy extension (pecl install mongo) and use PHP 5.x, or update your code to use this new driver's classes as the old driver is not available for PHP 7. There is an upgrade guide at http://mongodb.github.io/mongo-php-library/upgrade-guide/

Source : https://github.com/mongodb/mongo-php-driver/issues/300#issuecomment-210820288

Here is the replacement class documentation and the snippet of code that should replace yours :

$m = new MongoDB\Driver\Manager("mongodb://localhost:27017");

Another way, as suggested by the MongoDB member quoted here above is to use this pecl extension: https://pecl.php.net/package/mongo instead of https://pecl.php.net/package/mongodb but please also notice the warning there stating: