Laravel semicolon issue while dealing in query

I wanted to get the selected fields of the query from an outside array.

foreach($param as $key => $val){
            if($val == 'userId'){
                $string .= "adminusers.id, ";
            }
            if($val == 'name'){
                $string .= "CONCAT(firstName, ' ', lastName) as name";
            }
        }

My query is right below;

$where = '1';
$resultSet = UserAdmin::whereRaw($where)
                ->addSelect(array($string))
                ->groupBy('adminusers.id'); 

However, I received this :

SQLSTATE[42S22]: Column not found: 1054 Unknown column 'stmd_adminusers.id, CONCAT(firstName, ' ', lastName)' in 'field list' (SQL: select stmd_adminusers.id, CONCAT(firstName, ' ', lastName) as name from stmd_adminusers

via Chebli Mohamed