Transactions doesn't work correctly

Hi!!!
I'm trying to make a rollback after a query, but doesn't work...

if i execute this

$db_pdo->transaction("B");
for($i=0;$i<20000;$i++){
$db_pdo->query("INSERT INTO test_insert (name) VALUES ('Gino');");
}
$db_pdo->transaction("R");

it commits the query without rollback...
How can i fix it?

i made this modify in the class:
($this->con->query("SET ISOLATION TO SERIALIZABLE");)

public function transaction($type){
$this->err_msg = "";
if($this->con!=null){
try{
if($type=="B"){
$this->con->query("SET ISOLATION TO SERIALIZABLE");
$this->con->beginTransaction();
}
elseif($type=="C")
$this->con->commit();
elseif($type=="R")
$this->con->rollBack();
else{
$this->err_msg = "Error: The passed param is wrong! just allow [B=begin, C=commit or R=rollback]";
return false;
}
}catch(PDOException $e){
$this->err_msg = "Error: ". $e->getMessage();
return false;
}
}else{
$this->err_msg = "Error: Connection to database lost.";
return false;
}
}