DI

di.php

<?php

class Client
{
  private $service;
  
  public function __construct()
  {
    $this->service = new Service();
  }
  
  public function get()
  {
    $conn = $this->service->connect();
  }
}

class Service
{
  public function connect()
  {
  }
}

class Client
{
  private $service;
  
  public function __construct(Service $service)
  {
    $this->service = $service;
  }
}

class Client
{
  private $service;
  
  public function __construct()
  {
  }
  
  public function setService(Service $service)
  {
    $this->service = $service;
  }
}

class Client
{
  private $service;
  
  public function __construct(ServiceInterface $service)
  {
    $this->service = $service;
  }
}