Laravel5.2 SQL builder test condition to where query

I do search function In raw SQL like this

//test condition variable
 if ($param_type != "") {
    if($param_type =='SAS'){
        $str .= " AND pos.type IN('SAS1','SAS2') ";
    }
    else if($param_type =='SME'){
        $str .= " AND pos.type IN('SME1','SME2') ";
    }
    else if($param_type =='PT'){
        $str .= " AND pos.type IN('PT','PT0') ";
    }else{
        $str .= " AND pos.type ='$param_type'";
    }
}

//SQL

$query =" SELECT * FROM tbl_user AS u INNER JOIN tbl_position AS pos 
         ON u.user_id = pos.user_id where u.levels IN ('7','8','9') .$str ;" ;

How I can write SQL like this in laravel ?

via Chebli Mohamed