Dependency Inpervion Principle is one of the SOLID design principle. It states following two thing:

1. High-level modules should not depend on low-level modules. Both should depend on the abstraction.
2. Abstractions should not depend on details. Details should depend on abstractions.

interface LoggerInterface
{
    public function log(): string;
}
 
class FileLogger implements LoggerInterface
{
    public function log() : string
    {
        return "logged in a file";
    }
}
 
class StdoutLogger implements LoggerInterface
{
    public function log(): string
    {
        return "logged in screen";
    }
}
 
class LoggerManager
{
    private $logger;
 
    public function __construct(LoggerInterface $logger)
    {
        $this->logger = $logger;
    }
 
    public function log(): string
    {
        return $this->logger->log();
    }
}

Test cases

<?php declare(strict_types=1);
namespace App\Tests;
 
require_once './vendor/autoload.php'; 
use PHPUnit\Framework\TestCase;
use App\DIP\FileLogger;
use App\DIP\StdoutLogger;
use App\DIP\LoggerManager;
 
class DipTest extends TestCase
{
    public function testFileLogger()
    {
        $logger = new FileLogger();
        $loggerManager = new LoggerManager($logger);
 
        $this->assertRegExp('/logged in a file/', $loggerManager->log());
    }
 
    public function testStdoutLogger()
    {
        $logger = new StdoutLogger();
        $loggerManager = new LoggerManager($logger);
 
        $this->assertRegExp('/logged in screen/', $loggerManager->log());
    }
}
 
 

Execute test case

./vendor/bin/phpunit tests/DipTest.php PHPUnit 7.5.20 by Sebastian Bergmann and contributors.

.. 2 / 2 (100%)

Time: 30 ms, Memory: 4.00 MB

OK (2 tests, 2 assertions)