Description:
------------
Seems as if mysqlnd driver doesn't support PDO::MYSQL_ATTR_MAX_BUFFER_SIZE 
constant.

In pdo_mysql.c (from svn 
http://svn.php.net/repository/php/php-
src/branches/PHP_5_3/ext/pdo_mysql/pdo_mysql.c), I can see:

#ifndef PDO_USE_MYSQLND
	REGISTER_PDO_CLASS_CONST_LONG("MYSQL_ATTR_MAX_BUFFER_SIZE", 
(long)PDO_MYSQL_ATTR_MAX_BUFFER_SIZE);
...
#endif

So, this constant (and some others) are only defined when mysqlnd isn't the 
active driver.

Test script:
---------------
echo PDO::MYSQL_ATTR_MAX_BUFFER_SIZE;

Expected result:
----------------
Output 1005

Actual result:
--------------
Fatal error: Undefined class constant 'MYSQL_ATTR_MAX_BUFFER_SIZE' in php shell 
code on line 1