Problems

Coupling
Information Hiding
Code Duplication
Fail Fast
Integrity

Solutions

Reify objects
Create cohesive small objects
Don't leave them anaemic, find their cohesive relations.

Sample Code

Wrong

<?

$coordinate = array('latitude'=>1000, 'longitude'=>2000); 
//Its OK. they are just arrays. a Bunch of raw data

Anaemic

<?

final class GeographicCoordinate {

    function __construct($latitude, $longitude) {
        $this->longitude = $longitude;
        $this->latitude = $latitude;
    }
}

$coordinate = new GeographicCoordinate(1000, 2000);
//should throw an error since these values don't exist on Earth

Validated

<?

final class GeographicCoordinate {

    function __construct($latitude, $langitude) {
        if (!$this->isValidLatitude($latitude)) {
            throw new InvalidLatitudeException($latitude);
            //...
            $this->longitude = $longitude;
            $this->latitude = $latitude;
        }
    }
}

$coordinate = new GeographicCoordinate(1000, 2000);
//trows an error since these values don't exist on Earth

Right

Degrees deserves reification

<?

final class Latitude {
    function __construct($degrees) {
        if (!$degrees->between(-90, 90)) {
            throw new InvalidLatitudeException($latitude);
        }
        //...
    }
}

Many people suffer from primitive obsession and believe this is over design.
Designing software is about making decisions and comparing trade-offs.
The performance argument is not valid nowadays since modern virtual machines can efficiently deal with small short-lived objects.

<?

final class GeographicCoordinate {

    function distanceTo(GeographicCoordinate $coordinate) {
    }

    function pointInPoligon(Polygon $polygon) {
        //....
    }
}

//Now we are in geometry world (and not in array world anymore). we can safely do many exciting things.