app.php

<?php

namespace Dog\App;

use Dog\Dog;

$beethoven = new Dog("Beethoven");
$beethoven->bark();

dog.php

<?php

namespace Dog\Dog;

class Dog
{
  private $name;

  private static $dogCount = 0;

  function __construct($name = null)
  {
    $this->name = $name ?? "A Dog";
    self::$dogCount += 1;
  }

  function __toString()
  {
    return "<Dog:$this->name>";
  }

  function bark()
  {
    echo "$this->name goes bark!\n";
  }

  static function dogCount()
  {
    return self::$dogCount;
  }

  static function allDogsBark(...$dogs)
  {
    foreach($dogs as $dog)
    {
      $dog->bark();
    }
  }
}