Logger.php

<?php

interface Logger 
{
  public function log(string $message, int $level = 0): void;
}

Service.php

<?php

class Service 
{
  private $logger;
  public function __construct(Logger $logger) {
    $this->logger = $logger;
  }
  public function action() {
    $this->logger->log("start");
  }
}