Description:
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Currently, PDO::FETCH_FUNC can only be used in the PDOStatement::fetchAll() method. This fetch mode, however, is essentially useless since it can not be set using setFetchMode() or fetch(), and thus can not be used in iteration. 

Test script:
---------------
<?php
$db = new PDO(...);
$stmt = $db->execute("SELECT * FROM foobar");
$stmt->setFetchMode(PDO::FETCH_FUNC, 'var_dump');
foreach($stmt as $row):
    ...
endforeach;
?>

Expected result:
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PDO should set the fetch mode to FETCH_FUNC, and should call var_dump() when $stmt is iterated. Because no additional fetch modes were passed to setFetchMode(), var_dump() should receive an argument representing the row in PDO::FETCH_BOTH format. $row should be set to the return of var_dump(), and control should now be passed to the foreach codeblock.

IMHO, FETCH_FUNC should allow one to provide a callback function that allows full manipulation of the row before being passed into the iteration codeblock. For example, in an active record implementation, one would have to set the FETCH_CLASS method and suffer a very costly object instantiation. A callback function would allow me to clone an existing (and fully loaded) object, set my properties, and return it -- saving me upwards of 90% in execution costs for heavy objects.

Actual result:
--------------
Warning: PDOStatement::setFetchMode() [pdostatement.setfetchmode]: SQLSTATE[HY000]: General error: PDO::FETCH_FUNC is only allowed in PDOStatement::fetchAll()