I am working on saving state of an object to a database using the data mapper pattern, but I am looking for suggestions/guidance on the validation and error message generation step (step 4 below). Here are the general steps as I see them for doing this:

(1) The data mapper is used to get current info (assoc array) about the object

in db:

+=====================================================+  
|  person_id  |   name   |   favorite_color   |  age  |  
+=====================================================+  
|   1         |   Andy   |   Green            |  24   |   
+-----------------------------------------------------+

mapper returns associative array, eg. Person_Mapper::getPersonById($id) :

$person_row = array(
    'person_id' => 1,
    'name' => 'Andy',
    'favorite_color' => 'Green',
    'age' => '24',
);    

(2) the Person object constructor takes this array as an argument, populating its fields.

class Person {

    protected $person_id;
    protected $name;
    protected $favorite_color;
    protected $age;

    function __construct(array $person_row)
    {
        $this->person_id = $person_row['person_id'];
        $this->name = $person_row['name'];
        $this->favorite_color = $person_row['favorite_color'];
        $this->age = $person_row['age'];
    }

    // getters and setters...

    public function toArray()
    {
        return array(
            'person_id' => $this->person_id,
            'name' => $this->name,
            'favorite_color' => $this->favorite_color,
            'age' => $this->age,
        );
    }
}

(3a) (GET request) Inputs of an HTML form that is used to change info about the person is populated using Person::getters

<form>
    <input type="text" name="name" value="<?=$person->getName()?>" />
    <input type="text" name="favorite_color" value="<?=$person->getFavColor()?>" />
    <input type="text" name="age" value="<?=$person->getAge()?>" />
</form>

(3b) (POST request) Person object is altered with the POST data using Person::setters

$person->setName($_POST['name']);
$person->setFavColor($_POST['favorite_color']);
$person->setAge($_POST['age']);

*(4) Validation and error message generation on a per-field basis
- Should this take place in the person object or the person mapper object?
- Should data be validated BEFORE being placed into fields of the person object?

(5) Data mapper saves the person object (updates row in the database):

$person_mapper->savePerson($person);
// the savePerson method uses $person->toArray() 
// to get data in a more digestible format for the 
// db gateway used by person_mapper

Any guidance, suggestions, criticism, or name-calling would be greatly appreciated.