Basic ORM sketch that uses value objects, data mappers that persists data to a data source, and adapters for data sources.

Adapter.php

<?php
abstract class Adapter
{
    protected $config;
    protected $connection;
    
    public function __construct($config)
    {
        $this->config = $config;
        
        $this->connect();
    }
    
    public function __destruct()
    {
        $this->disconnect();
    }
    
    abstract public function connect();
    abstract public function disconnect();
    abstract public function insert($table, Entity $model);
    abstract public function update($table, Entity $model);
    abstract public function delete($table, Entity $model);
}

DataMapper.php

<?php
abstract class DataMapper
{
    protected $adapter;
    
    public function __construct(Adapter $adapter)
    {
        $this->adapter = $adapter;
    }
    
    public function save(Entity $model)
    {
        if (isset($model->id)) {
            return $this->adapter->update($this->table, $model);
        } else {
            return $this->adapter->insert($this->table, $model);
        }
    }
    
    public function delete(Entity $model)
    {
        return $this->adapter->delete($this->table, $model);
    }
}

Entity.php

<?php
abstract class Entity
{
    protected $dirtyFields = array();
    
    abstract public function getFields();
    
    public function getDirtyFields()
    {
        return $this->dirtyFields;
    }
    
    public function __set($key, $value)
    {
        $this->dirtyFields[$key] = $this->$key = $value;
    }
}

index.php

<?php
require('Entity.php');
require('User.php');

require('Adapter.php');
require('MySqlAdapter.php');

require('DataMapper.php');
require('UserMapper.php');

$user = new User();
$user->first_name = 'Martin';
$user->last_name = 'Bean';

$config = array(
    'host' => 'localhost',
    'username' => 'root',
    'password' => '',
    'dbname' => 'test_db'
);

$adapter = new MySqlAdapter($config);

$mapper = new UserMapper($adapter);

echo '<p>Creating user:</p>';

print '<pre>';
var_dump($mapper->save($user));
print '</pre>';

echo '<p>Deleting user:</p>';

print '<pre>';
var_dump($mapper->delete($user));
print '</pre>';
exit;

MySqlAdapter.php

<?php
class MySqlAdapter extends Adapter
{
    public function connect()
    {
        $dsn = sprintf('mysql:host=%s;dbname=%s', $this->config['host'], $this->config['dbname']);
        $driver_options = array(
            PDO::MYSQL_ATTR_INIT_COMMAND => 'SET NAMES utf8'
        );
        
        $this->connection = new PDO($dsn, $this->config['username'], $this->config['password'], $driver_options);
        $this->connection->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    }
    
    public function disconnect()
    {
        $this->connection = null;
    }
    
    public function insert($table, Entity $model)
    {
        $columns = implode(',', array_keys($model->getDirtyFields()));
        $values = rtrim(str_repeat('?,', count($model->getDirtyFields())), ',');
        
        $sql = "INSERT INTO `$table` ($columns) VALUES ($values)";
        
        $statement = $this->connection->prepare($sql);
        
        if (true === $statement->execute(array_values($model->getDirtyFields()))) {
            $model->id = (int) $this->connection->lastInsertId();
            return true;
        }
        
        return false;
    }
    
    public function update($table, Entity $model)
    {
        $id = (int) $model->id;
        $data = array();
        
        foreach ($model->getDirtyFields() as $column => $value) {
            $data[] = sprintf('`%s` = ?', $column);
        }
        
        $data = implode(',', $data);
        
        $sql = "UPDATE `$table` SET $data WHERE `id` = ? LIMIT 1";
        
        $statement = $this->connection->prepare($sql);
        
        $values = array_values($model->getDirtyFields());
        $values[] = $id;
        
        return $statement->execute($values);
    }
    
    public function delete($table, Entity $model)
    {
        $sql = "DELETE FROM `$table` WHERE `id` = ? LIMIT 1";
        
        $statement = $this->connection->prepare($sql);
        
        if (true === $statement->execute(array($model->id))) {
            $model->id = null;
            return true;
        }
        
        return false;
    }
}

User.php

<?php
class User extends Entity
{
    public function getFields()
    {
        return array(
            'id',
            'first_name',
            'last_name'
        );
    }
}

UserMapper.php

<?php
class UserMapper extends DataMapper
{
    protected $table = 'users';
}