The Mongo extension for PHP Version 5.99.99 or older has been superseded:
https://pecl.php.net/package/mongo
Use the newer one for PHP Version 7.99.99 or older instead:
https://pecl.php.net/package/mongodb
You can install a PECL/PEAR extension automatically:
pecl install mongodb
or manually.
The classes have been changed too:
new \MongoClient(); // legacy class!
see http://php.net/manual/en/book.mongo.php
new \MongoDB\Driver\Manager(); // new classes!
see http://php.net/manual/en/set.mongodb.php
Additional information regarding compatibility can be found here:
https://docs.mongodb.org/ecosystem/drivers/php/#compatibility