Laravel4.2和MongoDB的。 尝试从命令行调用命令时找不到类“MongoClient”

As a Topic title. I work with cron service to execute artisan command every minute. I have a problem when I try to execute command via terminal for debuging. I got error Class 'MongoClient' not found.

So I try to do simple query in laravel Route

$m  = new MongoClient('mongodb://localhost:27017');
$db = $m->rootan;
print_r($db) ;
$result = $db->campaigns->findOne();
print_r($result) ;

I got a result with no error perfectly.

So my question is why I get this error when execute command from terminal. Do this problem is something about Permission?

Sorry for my english.

PS. I use Ubuntu14.04 with Laravel4.2 and MongoDB3