以下是一个使用PHP和MongoDB进行对象文档映射（ODM）的简单实例。我们将创建一个简单的用户模型，并展示如何使用ODM进行增删改查操作。
用户模型
```php
class User
{
private $id;
private $username;
private $email;
public function __construct($username, $email)
{
$this->username = $username;
$this->email = $email;
}
// Getter 和 Setter 方法
public function getId()
{
return $this->id;
}
public function getUsername()
{
return $this->username;
}
public function setUsername($username)
{
$this->username = $username;
}
public function getEmail()
{
return $this->email;
}
public function setEmail($email)
{
$this->email = $email;
}
}
```
数据库连接
```php
class Database
{
private static $instance = null;
private $mongo;
private function __construct()
{
$this->mongo = new MongoDB""Client("
