Refactoring: Push down method

Previously we mentioned the pull up method we can use during refactoring our code. The push down method is the exact opposite. Let's start with some code:

abstract class animal {

  function bark(){
    // bark here
  }

}

class dog extends animal{}

class cat extends animal{}

$dog = new dog();
$dog->bark();

Maybe one day our cat had a bark about it, or maybe we've coded an if statement to do something different like return false if it's a cat. 

abstract class animal {}
class dog extends animal {
  function bark(){
    // bark here
  }
}
class cat extends animal {}

$dog = new dog();
$dog->bark();