Commit and Rolback Mysqli

Question

Guys, I’m using PHP’s Mysqli class and I came across the following situation...

I have two tables, the table A is the main and the table B is the daughter until there is ok, what I need to do is commit treatment and rollback to avoid failures in the future this way I put together the following script...

$DB->mConnect();
$DB->Mysqli->autocommit(false);

$er = true;

$query = "insert into (descricao) values ('testes')";
$er = !$DB->Mysqli->query($query) ? false : true;
$codigo = $DB->Mysqli->insert_id;

$query2 = "insert into (cod_pai, descricao) values (".$codigo.", 'teste filho')";
$er = !$DB->Mysqli->query($query2) ? false : true;

$er ? $DB->Mysqli->commit() : $DB->Mysqli->rollback();

var_dump($err);

Now comes my question, the daughter table B is dependent on the auto increment code that is generated in table A, what is occurring is that the way I did is not being generated the Code, and this way when will use in table B tells that the code of table A does not exist.

I wish I knew what I did wrong?

I appreciate help!