新版本的pdo會有這個問題

新版本的pdo會有這個問題： 

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。。。 

意思是要麼使用fetchAll(),要麼使用MYSQL_ATTR_USE_BUFFERED_QUERY這個屬性，前者不想那樣用，因爲擔心佔用內存，後者發現沒效果。 

 $this->db->setAttribute(PDO::MYSQL_ATTR_USE_BUFFERED_QUERY, true);