Interface Segregation Principle (ISP)

InterfaceSegregation.php

<?php

$bird = new Parrot();

$bird->fly();

interface BirdInterface
{
    public function fly();
}

class Parrot implements BirdInterface
{
    public function fly()
    {
        // ...fly away, Peter!
    }
}

class Penguin implements BirdInterface
{
    // ...don't fly
}