PHP interface Keyword

<?php
interface Machine {
  public function activate();
  public function deactivate();
  public function isActive();
}

class Kettle implements Machine {
  private $isOn = false;

  public function activate() {
    $this->isOn = true;
  }

  public function deactivate() {
    $this->isOn = false;
  }

  public function isActive() {
    return $this->isOn;
  }
}

$machine = new Kettle();

$machine->activate();
if($machine->isActive()) {
  echo "The machine is on";
} else {
  echo "The machine is off";
}

echo "<br>";
$machine->deactivate();
if($machine->isActive()) {
  echo "The machine is on";
} else {
  echo "The machine is off";
}
?>