mysql pdo 事务嵌套

<?php

class database extends PDO
{
protected $_transTimes = 0;

public function beginTransaction()
{
++$this->_transTimes;
if ($this->_transTimes == 1) {
return parent::beginTransaction();
}

$this->exec('SAVEPOINT trans' . $this->_transTimes);
return $this->_transTimes >= 0;
}

public function commit()
{
--$this->_transTimes;
if ($this->_transTimes == 0) {
return parent::commit();
}

return $this->_transTimes >= 0;
}

public function rollback()
{
--$this->_transTimes;
if ($this->_transTimes == 0) {
return parent::rollback();
}

$this->exec('ROLLBACK TO trans' .$this->_transTimes + 1);
return true;
}
}