Dependency Injection

di.php

<?php

class Database
{
	protected $adapter;

	public function __construct()
	{
		$this->adapter = new MysqlAdapter;
	}
}

class MysqlAdapter
{

}

// using DI
class Database
{
	protected $adapter;

	public function __construct(MysqlAdapter $adapter)
	{
		$this->adapter = $adapter;
	}
}

// Using D in SOLID "Depends on Abstraction, not depend on concretions"
class Database
{
	protected $adapter;

	public function __construct(AdapterInterface $adapter)
	{
		$this->adapter = $adapter;
	}
}

interface AdapterInterface {}
class MysqlAdapter implements AdapterInterface
{

}