abstract.php

<?php

abstract class Shape
{
  public function isShape()
	{
		return true;
	}
}

class Square extends Shape
{
	public function isSquare()
	{
		return true;
	}
}

class Circle extends Shape
{
	public function isSquare()
	{
		return false;
	}
}

class Person
{
	public function isSquare()
	{
		return false;
	}
}

function checkSquare (Shape $sh) {
	echo ($sh->isSquare()) ? "Yep" : "No";
}

  checkSquare(new Square); // <- echoes "Yep"
//checkSquare(new Circle); // <- echoes "No"
//checkSquare(new Person); // <- type error

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