运行环境：PHP 5.5.30-x64，MYSQL 5.6.27
错误代码：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.
网上的解释：
经过测试发现是因为在前一次 execute 里面执行了多条SQL语句（即通过分号分隔连续执行了多次的select/insert/update）。也就是说，未结束之前的查询而再次查询，这样是会出错地，所以要释放掉之前的查询。
手册上的代码：
<?php
if ( $db -> getAttribute ( PDO :: ATTR_DRIVER_NAME ) == 'mysql' ) {
$stmt = $db -> prepare ( 'select * from foo' ,
array( PDO :: MYSQL_ATTR_USE_BUFFERED_QUERY => true ));
} else {
die( "my application only works with mysql; I should use \$stmt->fetchAll() instead" );
}
//http://www.cnblogs.com/osfipin/
?>
If this attribute is set to TRUE on a PDOStatement, the MySQL driver will use the buffered versions of the MySQL API. If you're writing portable code, you should use PDOStatement::fetchAll() instead.
测试了很多次还是有问题。。。。。
代码运行多个query，OPTIMIZE TABLE，DELETE操作。发现是OPTIMIZE TABLE导致的此错误。注释掉后代码运行无误。
使用OPTIMIZE TABLE：
<?php
class database extends PDO{
//表前缀
public $prefix = ''; //一些代码 //优化表
function optimize($table){
$stmt = $this -> prepare ('OPTIMIZE TABLE `'.$this->prefix.$table.'`',array( PDO :: MYSQL_ATTR_USE_BUFFERED_QUERY => true ));
return $stmt -> execute();
} //一些代码
}
啦啦啦。。。