abstract_vs_interface

interface MapperInterface
{
  public function map();
}

abstract class BaseUserMapper
{
  public function __construct($userEntity)
  {
    $this->user = $userEntity;
  }
  
  abstract public function map();

  public function toArray()
  {
    return [
      'id' => $this->comment->getId(), 
      'username' => $this->comment->getUsername()
    ];
  }
}

abstract class BaseCommentMapper
{
  public function __construct($commentEntity)
  {
    $this->comment = $commentEntity;
  }
  
  abstract public function map();
  
  public function toArray()
  {
    return [
      'id' => $this->comment->getId(), 
      'text' => $this->comment->getText()
    ];
  }
}

class JsonUserMapper extends BaseUserMapper implements MapperInterface
{
  public function map()
  {
    // do user specific mapping stuff for json files
    return json_decode($this->toArray());
  }
}

class JsonCommentMapper extends BaseCommentMapper implements MapperInterface
{
  public function map()
  {
    // do comment specific mapping stuff for json files
    return json_decode($this->toArray());
  }
}

class Writer
{
    public function write(MapperInterface $mapper)
    {
        file_put_contents('output.file', $this->mapper->map());
    }
}

$writer = new Writer();

// does matter which mapper you use; the MapperInterface makes sure there is always a map method
$writer->write(new JsonCommentMapper($comment));