PDO: transazione e exception ?

[dinopc]

Salve, ho un dubbio su come funziona la classe PDO e in particolare la gestione delle eccezioni e delle transazioni.

In giro ho trovato il seguente script:

<?php
/*** mysql hostname ***/
$hostname = 'localhost';

/*** mysql username ***/
$username = 'username';

/*** mysql password ***/
$password = 'password';

/*** database name ***/
$dbname = 'dbname';

try {
    $options = array(PDO::ATTR_AUTOCOMMIT=>FALSE);
    $dbh = new PDO("mysql:host=$hostname;dbname=$dbname", $username, $password, $options);
    /*** echo a message saying we have connected ***/
    echo 'Connected to database
';

    /*** set the PDO error mode to exception ***/
    //$dbh->setAttribute(PDO_ATTR_AUTOCOMMIT, 0); 
    $dbh->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

    /*** CREATE table statements ***/
    $table = "CREATE TABLE animals ( animal_id MEDIUMINT(8) NOT NULL PRIMARY KEY,
    animal_type VARCHAR(25) NOT NULL,
    animal_name VARCHAR(25) NOT NULL 
    )";
    $dbh->exec($table);

    /*** begin the transaction ***/
    $dbh->beginTransaction();

    /***  INSERT statements ***/
    $dbh->exec("INSERT INTO animals (animal_id, animal_type, animal_name) VALUES (1, 'emu', 'bruce')");
    $dbh->exec("INSERT INTO animals (animal_id, animal_type, animal_name) VALUES (2, 'funnel web', 'bruce')");
    $dbh->exec("INSERT INTO animals (animal_id, animal_type, animal_name) VALUES (3, 'lizard', 'bruce')");
    $dbh->exec("INSERT INTO animals (animal_id, animal_type, animal_name) VALUES (4, 'dingo', 'bruce')");
    $dbh->exec("INSERT INTO animals (animal_id, animal_type, animal_name) VALUES (5, 'kangaroo', 'bruce')");
    $dbh->exec("INSERT INTO animals (animal_id, animal_type, animal_name) VALUES (6, 'wallaby', 'bruce')");
    $dbh->exec("INSERT INTO animals (animal_id, animal_type, animal_name) VALUES (6, 'wombat', 'bruce')");
    $dbh->exec("INSERT INTO animals (animal_id, animal_type, animal_name) VALUES (8, 'koala', 'bruce')");
    $dbh->exec("INSERT INTO animals (animal_id, animal_type, animal_name) VALUES (9, 'kiwi', 'bruce')");

    /*** commit the transaction ***/
    $dbh->commit();

    /*** echo a message to say the database was created ***/
    echo 'Data entered successfully
';
}
catch(PDOException $e)
    {
    /*** roll back the transaction if we fail ***/
    $dbh->rollback();

    /*** echo the sql statement and error message ***/
    echo $sql . '
' . $e->getMessage();
    }
?>

mi aspettavo che nella tabella in oggetto 'animals' non fossero presenti nessun record inserito perchè avendo violato la chiave primaria 'animal_id = 6' e non avendo fatto la commit.

Cosa sbaglio ?