Model class : how to build a model using PDO's prepared statements syntax?

I want to build a model class for my PHP application. It will have methods meant to select/update/insert/delete specific data from a database according to the method's parameters. I only want to use prepared statements.

Here is an overview of what the class should look like :

class Database {

    private $_db;

    // Stores a PDO object (the coection with the database) within the $_db property
    public function __construct($host, $user, $password) {...}

    public function select() {...}

    public function update() {...}

    public function insert() {...}

    public function delete() {...}

}

The problem is that I don't really know how to do this. Let's say I want to select everything from the table "farm" where the animal is a dog. The syntax for this statement would be the following :

$animal = $dog;
$query = $this->_db->prepare('SELECT * FROM farm WHERE animal = :animal');
$query->execute(array(':animal' => $animal));

$result_set = $query->fetchAll();

This is very complicated to implement within a class method. As you can see, I call the execute() method but I don't even know in advance if the WHERE clause will be used !

And even worse : what if I will want to use, let's say, the LIMIT x, y clause later on ?

Which parameters should I ask for and how to treat them ? Should I simply require the parameters to be one query + multiple variables that will be passed to the execute() method ?

Are these types methods reasonable for what I want to do ?

Thanks in advance :P