PDO执行execute方法后无任何返回，通过try catch 得到如下异常信息

SQLSTATE[HY000]: 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.

解决方法：

$stmt->closeCursor();