loggable.php

<?php

namespace Psr\Log;

interface Loggable
{
  public function getContext(); 
}

class Car implements Loggable
{
  public function getContext()
  {
    return array(
      'plate_number'  => $this->getPlateNumber(),
      'model'         => $this->getModel(),
    );
  }
}

// userland code

if ($car->turnsOn()) {
  $logger->info("Hey today the car turned on!", $car);
} else {
  $logger->err("Hmmm...", $car);
}