How to fetch all the fields in mongodb aggregate function? - mongodb

Question

I have wriiten below lines of code
public function BasicSearchStudents($input)
{
$this->page =1;
$page=(int)$this->page;
$skip=($page - 1)*12;
$param = preg_replace('!\s+!', ' ', $input);
$arg = trim($param);
$output= $this->db->studentTbl->aggregate(
array(
array(
'$project' => array(
'fullname' => array('$concat' => array('$first_name', ' ', '$middle_name', ' ', '$last_name')),
'firstmiddle' => array('$concat' => array('$first_name', ' ', '$middle_name')),
'firstlast' => array('$concat' => array('$first_name', ' ', '$last_name')),
'first_name' => 1,
'middle_name' =>1,
'last_name' =>1,
'roll_no' =>1,
'registration_temp_perm_no' =>1,
'email' =>1,
'guardian_name' =>1,
'phone1' =>1,
'phone2' =>1,
'dob' =>1,
'student_id' =>1,
'is_active' =>1,
'has_finished' =>1
)
),
array(
'$match' =>
array('$or' =>
array(
array("fullname" => new MongoRegex("/$arg/i")),
array("first_name" => new MongoRegex("/$arg/i")),
array("middle_name" => new MongoRegex("/$arg/i")),
array("last_name" => new MongoRegex("/$arg/i")),
array("firstmiddle" => new MongoRegex("/$arg/i")),
array("firstlast" => new MongoRegex("/$arg/i")),
array("registration_temp_perm_no" => new MongoRegex("/$arg/i")),
array("roll_no" => new MongoRegex("/$arg/i")),
array("email" => new MongoRegex("/$arg/i")),
array("guardian_name" => new MongoRegex("/$arg/i")),
array("phone1" => new MongoRegex("/$arg/i")),
array("dob" => new MongoRegex("/$arg/i"))
)
)
),
array(
'studentresult' => array(
'$push' => '$$ROOT'
)
),
array('$limit' => 22),
array('$skip' =>$skip),
)
);
$cursor = $output["studentresult"];
}
I am trying to fetch all the fields used in student collection in by using root after match operation. It throws error "Uncaught exception 'MongoResultException' with message 'localhost:27017: Unrecognized pipeline stage name: 'result''".
Please help me.