memeloper.php

<?php 
# copy right Fermin Perdomo
interface day {
	
	public function whatYouDoing();
}

class Holiday implements day {
	
	protected $dayName = "";
	
	public function __construct($dayName)
	{
		$this->dayName = $dayName;
	}
	
	public function whatYouDoing()
	{
		return "Es {$this->dayName}, Búscame en la fiesta!";
	}
}

class RegularDay implements day {
	
	public function whatYouDoing()
	{
		return "Estoy programando";
	}
	
}

$days = [new RegularDay(), new RegularDay(), new RegularDay(), new Holiday("31 de Diciembre"), new Holiday("01 de Enero")];
foreach($days as $day){ echo $day->whatYouDoing().PHP_EOL; }