very-simple-oop-plugin.php

<?php

defined('NDA') || exit('Access denied.');

/**
 * Plugin Name: OOP Plugin
 * Description: A very basic plugin using OOP (Interface, Abstract Class, Class and Function).
 * Version: 2018.08.24
 * License: GPLv2+
*/

interface iInterface
{
	public function echoHello();

	public function returnHello();
}

abstract class aClass implements iInterface
{
	public function returnHello()
	{
		return "Hello";
	}
}

class cClass extends aClass
{
	public function echoHello()
	{
		$hello = aClass::returnHello();
		echo $hello;
	}
}

$oop = new cClass();
$oop->echoHello();