blou.php

<?php

namespace foo;

class message
{
 …
}

namespace ubermuda;

interface message
{
  function ifIsSigned(callable $callable) :void;
}

namespace ubermuda\message;

use ubermuda\message;

class foo extends \foo\message
  implements
    message  
{
  function ifIsSigned(callable $callable) :void
  {
    if ($message->getProperty(\foo\MessageBuilder::PROPERTY_IS_SIGNED, false))) {
      $callable();
    }
  }
}

namespace ubermuda;

class bar
{
  function doSomethingWithMessage(message $message) :void
  {
    $message->ifIsSigned(function() {
        echo "Go nuts!";
      }
    );
  }
}