Yii2:- update record using update query for mongodb -

i trying make update query using yii2 mongodb. make following query:

$collection = yii::$app->mongodb->getcollection('usermaster');        $arrupdate = [              'firstname' => $fname,             'lastname' => $lname,             'email' => $email,             'is_visible' => $isvisibleuser,             'phonenumber' => $phone,             'username' => $uname,                ];    $collection->update(['_id = 55a4957sd88423d10ea7c07d'],$arrupdate);  

but shows follwoing error in firebug.

"networkerror: 500 internal server error - http://localhost/yii2angularseedmaster/frontend/web/category/corporateupdate?corpuserid=55a4957sd88423d10ea7c07d" 

my insert query works update query not works.

please tell me wrong in query.

$collection->update(['_id' => $id],$arrupdate);