I am trying to assign value to the variable '@month_count'. But laravel throws "error on query"

$this->src("mysql")
         ->query("set @month_count = :periodicity_id;")        //I want to assign 'periodicity_id' to '@month_count' variable
         ->params(array(":periodicity_id"=>$this->params["periodicity"]))
         ->saveTo($node);

I directly added the parameter instead of assigning to @month_count variable