<?php

interface Talker
{
  public function talk();
}

class HelloTalker implements Talker
{
  private $_listener;

  public function __construct($listener)
  {
    $this->_listener = $listener;
  }

  public function talk()
  {
    return sprintf('Hello %s!', $this->_listener);
  }
}

$hello = new HelloTalker('World');

echo $hello->talk();