PHP: one PDO instance breaks transaction of other PDO instance - php

Question

The code below works and although while having a running transaction I create another PDO object, this does not disturb my transaction:
$dsn = 'mysql:host=' . "localhost" . ';dbname=' . "somedb";
$options = array(
PDO::ATTR_PERSISTENT => true,
PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
PDO::ATTR_ORACLE_NULLS => PDO::NULL_TO_STRING
);
// first PDO object
$db = new \PDO($dsn, "someuser", "somepass", $options);
$db->beginTransaction();
$stmt = $db->prepare("INSERT INTO vbclient set company = 'test'");
$stmt->execute();
// second PDO object
$dbnew = new \PDO($dsn, "someuser", "somepass", $options);
$newstmt = $dbnew->prepare('select * from vbclient where clientID = 1');
$newstmt->execute();
$stmt = $db->prepare("INSERT INTO vbclient set company = 'test2'");
$stmt->execute();
$db->commit();
But if I put the creation of the second PDO object into a seperate function it seems to kill the transaction of the first PDO object:
$dsn = 'mysql:host=' . "localhost" . ';dbname=' . "somedb";
$options = array(
PDO::ATTR_PERSISTENT => true,
PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
PDO::ATTR_ORACLE_NULLS => PDO::NULL_TO_STRING
);
// first PDO object
$db = new \PDO($dsn, "someuser", "somepass", $options);
$db->beginTransaction();
$stmt = $db->prepare("INSERT INTO vbclient set company = 'test'");
$stmt->execute();
secondPDO();
$stmt = $db->prepare("INSERT INTO vbclient set company = 'test2'");
$stmt->execute();
$db->commit();
function secondPDO() {
$dsn = 'mysql:host=' . "localhost" . ';dbname=' . "somedb";
$options = array(
PDO::ATTR_PERSISTENT => true,
PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
PDO::ATTR_ORACLE_NULLS => PDO::NULL_TO_STRING
);
// second PDO object
$dbnew = new \PDO($dsn, "someuser", "somepass", $options);
$newstmt = $dbnew->prepare('select * from vbclient where clientID = 1');
$newstmt->execute();
}
I get the error "There is no active transaction." I understand that this normally happens if I overwrite the PDO handle with another instance. But I don't do that and the error only occurs if the second PDO object is in another function.
Any ideas why this is?
Thanks.