Understanding dependency injection

Throughout the introductory section, we touched upon passing dependency through the class __construct() method. There is more to it than just passing the dependent object. Let's consider the following three seemingly similar but different examples.

Though PHP has been supporting type hinting for quite a while now, it isn't uncommon to come across pieces of code, which are as follows:

<?php

class App
{
    protected $config;
    protected $logger;

    public function __construct($config, $logger)
    {
        $this->config = $config;
        $this->logger = $logger;
    }

    public function run()
    {
        $this->config->setValue('executed_at', time());
        $this->logger->log('executed');
    }
}

class Config
{
    protected $config = [];

    public function setValue($path, $value)
    {
        // implementation
    }
}

class Logger
{
    public function log($message)
    {
        // implementation
    }
}

$config =...