Description:
------------
When using multiple statments in a single $pdo->execute(),
php throws an exception, however the staments bieng executed anyway.
( example: http://pastebin.com/f6aec645c )

I think the mysql server expects the resultset to be advanced even if it's empty, as shown in the c-api example here:
http://dev.mysql.com/doc/refman/5.1/en/c-api-multiple-queries.html

I think the follwing will resolve the issue
( diff against svn 5_2 )
*** mysql_driver.c      Sat Nov 28 13:13:07 2009
--- /home/yury/php-5.2.11/ext/pdo_mysql/mysql_driver.c  Fri Nov 27 13:55:46 2009
***************
*** 252,261 ****
--- 252,262 ----
                my_ulonglong c = mysql_affected_rows(H->server);
                if (c == (my_ulonglong) -1) {
                        pdo_mysql_error(dbh);
                        return (H->einfo.errcode ? -1 : 0);
                } else {
+                       while ( 0 == mysql_next_result( H->server ) );
                        return c;
                }
        }
  }


Reproduce code:
---------------
http://pastebin.com/f6aec645c

Expected result:
----------------
throw exception and not commit
-or-
commit and not throw exception

Actual result:
--------------
The follwing exception thrown:

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.

But the data IS commited to the database.