Description:
------------
[Note: I am actually testing this on PHP 5.1.1.  If this bug was fixed in PHP 5.1.2, please add a note to the manual page for PDO::exec()].

The manual says:

"PDO::exec() does not return results from a SELECT statement. For a SELECT statement that you only need to issue once during your program, consider issuing PDO::query()."

Either the manual needs to be changed, or, ideally, PDO::exec() needs to be fixed to discard results.

This issue has bit me multiple times.  It's easy to forget that a certain query (such as MySQL's OPTIMIZE TABLE) will return a result.  Using PDO::exec() in such cases causes an error later that can be difficult to track down.

Reproduce code:
---------------
<?
     $db = new PDO("mysql:host=localhost;dbname=test");

     $db->exec("SELECT 1");

     $st = $db->prepare("SELECT NOW()");
     if ($st === false)
     {
         $e = $db->errorInfo();
         echo "$e[0]:$e[1]: $e[2]\n";
     }
?>


Expected result:
----------------
[nothing]

Actual result:
--------------
HY000:2014: 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.