Fatal error: Call to undefined function FollowYou() in C:\wamp\www\oop-beg\ch4-6.php on line 23

<?php
class Animal{}
class Person{}
class GrizzlyBear extends Animal implements IThreat{
  function GrowlAtYou(){
    echo "Growwwwwwwwl!!";
  }
  function ChaseYou(){}
  function PounceOnYourCar(){}
  function EatYou(){}
  function BeThreatening(){
    GrowlAtYou();
    ChaseYou();
    PounceOnYourCar();
  }
}
class Cop extends Person implements IThreat{
  function FollowYou(){}
  function PullYouOver(){}
  function ArrestYou(){}
  function BeatYouSenseless(){}
function BeThreatening(){
  FollowYou();
  PullYouOver();
  ArrestYou();
}
}
interface IThreat{
  public function BeThreatening();
}
class ScaryRoad{
  public function __construct(){}
  public function DriveCar(array $threatening_object){
    //while driving down a scary road
    foreach ($threatening_object as $threat){
      $threat->BeThreatening();
    }
  }
}
//$cop1=new Cop();
$cop1=new Cop();
$grizzly1 = new GrizzlyBear();
$grizzly2 = new GrizzlyBear();

$road=new ScaryRoad();
$road->DriveCar($threats=array($cop1,$grizzly1,$grizzly2));

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