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PDO::FETCH_CLASS with multiple classes
$result->setFetchMode(PDO::FETCH_CLASS, 'myclass', array());

ID # name # age
1  # jon  # 12
2  # sue  # 23
3  # tom  # 24

/**
 * Should not have any private, public or protected members in it's definition.
 *
 * Does only work for public properties.
 */
class ReturnObject {
    public function getConcrete()
    {
        /* decide here which class */
       $classname = 'Child'; // or 'Adult'

       return $this->selfAs($classname);
    }

    private function selfAs($classname)
    {
        $l = strlen(__CLASS__);
        $s = sprintf('O:%d:"%s"', strlen($classname), $classname).substr(serialize($this), 5+strlen($l)+$l);
        $instance = unserialize($s);
        $instance->__construct();
        return $instance;
    }
}

SELECT
id,
name,
age,
CASE WHEN age < 21 THEN 'child'
ELSE 'adult' END
AS class_name
FROM SOME_TABLE";

Then use your PDO::FETCH_ASSOC as fetch type
Then when you loop over your results
you can easily instantiave class based on
$result['class_name'] like this:
$class = $result['class_name'];
$obj = new $class;