A model implementation using the repository pattern and entity caching. From the article: PDO for Elegant PHP Database Access, http://www.modernphpbook.com/articles/pdo-for-elegant-php-database-access - Fig 5

UserRepository.php

<?php 
class UserRepository
{
    private $userCache = array();

    public function find($id)
    {
        if (!isset($this->userCache[$id])) {
            $userCache[$id] = $this->dao->query('
                SELECT * FROM users WHERE id = ' . (int)$id . '
            ');
        }
        return $userCache[$id];
    }
    // etc...
}