PHP strategy pattern - interface vs inheritance?

I am stuck here at whether should I use interface or not. For instance,

interface ModelStrategy 
{
   public function respond(); 
}

interface Mammal
{
    public function walk(Model $Model);
}

class Model implements ModelStrategy 
{
    public $text = null;

    public function respond() {
        return $this->text;
    }
}

class CatModel extends Model implements ModelStrategy 
{
    public $text = 'Walk using four legs or whatever';

    public function respond() {
        return $this->text;
    }
}

class Cat implements Mammal
{
    public function walk(Model $Model)
    {
        return $Model->respond();
    }
}

usage,

$mammal = new Cat;

function performWalk(Mammal $mammal){
    $model = new CatModel();
    return $mammal->walk($model);
}

$walk = performWalk($mammal);

var_dump($walk);

I can archive the same result above just with inheritance without interface, for instance,

class Model 
{
    public $text = null;

    public function respond() {
        return $this->text;
    }
}

class CatModel extends Model
{
    public $text = 'Walk using four legs or whatever';

    public function respond() {
        return $this->text;
    }
}

class Mammal
{
    public function walk(Model $Model)
    {
        return $Model->respond();
    }
}

class Cat extends Mammal
{
    public function walk(Model $Model)
    {
        return $Model->respond();
    }
}

It seems to be a lot simpler with inheritance, what would you do and what do you suggest?