<?php
abstract class template_ABC
{
	protected $data;

	abstract protected function foo();

	public function showData()
	{
		return $this->data;
	}
}

class a extends template_ABC
{
	public function foo($data)
	{
		$this->data = $data;
	}
}

class b extends a
{
	public function foo($data)
	{
		$this->data = $data;
	}
}

class c extends b
{
	public function foo($data)
	{
		$this->data = $data;
	}
}

$object = new a();
$object->foo('test');
echo $object->showData();
?>