新版本的pdo会有这个问题：
General error: 2014 Cannot execute queries while other unbuffered queries are active. Consider using PDOStatement::fetchAll(). Alternatively, if your code is only ever going to run against mysql, you may enable query buffering by setting the PDO::MYSQL_ATTR_USE_BUFFERED_QUERY attribute.' in。。。
意思是要么使用fetchAll(),要么使用MYSQL_ATTR_USE_BUFFERED_QUERY这个属性，前者不想那样用，因为担心占用内存，后者发现没效果。
<span>$this</span>->db->setAttribute(PDO::MYSQL_ATTR_USE_BUFFERED_QUERY, <span>true</span>);
参考博客：
http://www.flashj.cn/wp/pdo-transaction-err-in-php.html
