$sql = "DROP TRIGGER BeforeDeleteMoneyLog;";
$m->execute($sql);
报错：

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.
[ SQL语句 ] : DROP TRIGGER BeforeDeleteMoneyLog;
经检查 BeforeDeleteMoneyLog这个触发器确实存在，用phpmyadmin运行DROP TRIGGER BeforeDeleteMoneyLog;没有报错，
用thinkphp 版本为3.2.3，mysql版本为5.5.8，php版本为5.3.3