solid

Circle.php

class Circle
{
    public $radius;

    public function __construct($radius): void
    {
        $this->radius = $radius;
    }
}

class Square
{
    public $length;

    public function __construct($length): void
    {
        $this->length = $length;
    }
}

class SumCalculator
{
    public function sum(): float
    {
        $area = [];
        foreach($this->shapes as $shape) {
            if($shape instanceof Square){
                $area[] = pow($shape->length, 2);
            }else if($shape instanceof Circle) {
                $area[] = pi() * pow($shape->radius, 2);
            }
        }

        return array_sum($area);
    }
}

CirecleRefactored.php

class Circle
{
    public $radius;

    public function __construct($radius): void
    {
        $this->radius = $radius;
    }

    public function area(): float
    {
        return pi() * pow($this->radius, 2);
    }
}

class Square
{
    public $length;

    public function __construct($length): void
    {
        $this->length = $length;
    }

    public function area(): float
    {
        // logic
    }
}

class SumCalculator
{
    public function sum(): float
    {
        $area = [];
        foreach($this->shapes as $shape) {
            $area[] = $shape->area();
        }

        return array_sum($area);
    }
}

DIC.php

class PasswordReminder {
    private $dbConnection;

    public function __construct(MySQLConnection $dbConnection) 
    {
        $this->dbConnection = $dbConnection;
    }
}

interface DBConnectionInterface
{
    public function connect(): void;
}

class MySQLConnection implements DBConnectionInterface
{
    public function connect(): void 
    {
        return "Database connection";
    }
}

class PasswordReminder {
    private $dbConnection;

    public function __construct(DBConnectionInterface $dbConnection) 
    {
        $this->dbConnection = $dbConnection;
    }
}

Exam.php

class ExamService2
{
    public function resolveExam(Exam $exam, array $answers): void
    {
        // logic
    }
}

ExamExproterInerface.php

interface ExamExporterInterface interface
{
    public function export(Exam $exam): string;
}

class ExamXmlExporter implements ExamExporterInterface
{
    public function export(Exam $exam): string
    {
        // logic
    }
}

class ExamJsonExporter implements ExamExporterInterface
{
    public function export(Exam $exam): string
    {
        // logic
    }
}

class ExamCommand
{
    public function execute()
    {
        // logic getting exam
        $examService->resolveExam($exam, $answers);
        $examExporter->export($exam);
    }
}

ExamService.php

class ExamService
{
    public function resolveExam(Exam $exam, array $answers): void
    {
        // logic
    }

    public function export(Exam $exam): string
    {
       // logic
    }
}

class ExamCommand
{
    public function execute(): void
    {
        // logic getting exam
        $examService->resolveExam($exam, $answers);
        $examService->export($exam);
    }
}

ExamService2.php

class ExamService2
{
    public function resolveExam(Exam $exam, array $answers): void
    {
        // logic
    }
}

HttpProxy.php

interface HttpClientInterface
{
    public function get($url): Response;
}

class HttpClient implements HttpClientInterface
{
    public function get($url): Response
    {
        return new Response();
    }
}

class CachedHttpClient implement HttpClientInterface
{
    protected $httpClient;

    protected $cached = [];

    public function __constructor(HttpClientInterface $httpClient)
    {
        $this->httpClient = $httpClient;
    }

    public function get($url): Resint
    {
        return $this->cached[$url];
    }
}

Liskov.php

class Rectangle
{
    protected $witdth;

    protected $height;

    public function setWidth(int $width): void
    {
        $this->width = $width;
    }

    public function getWidth(): int
    {
        return $this->width;
    }

    public function setHeight(int $height): void
    {
        $this->height = $height;
    }

    public function getHeight(): int
    {
        return $this->height;
    }

    public function area(): int
    {
        return $this->width*$this->height;
    }
}

class Square extends Rectangle
{
    public function setWidth(int $width): void
    {
        parent::setWidth($width);
        parent::setHeight($width);
    }

    public function setHeight(int $height): void
    {
        parent::setHeight($height);
        parent::setWidth($height);
    }
}

class AreaTest extends \PHPUnit_Framework_TestCase
{
    public function testArea(): void
    {
        $rectangle = new Rectangle(); 
        $rectangle->setWidth(5);
        $rectangle->setHeight(4);

        $this->assertEquals(20, $rectangle->area());
    }
}

Segregacion.php

interface Product
{
  public function getName(): string;
  public function getStock(): int;
  public function getNumberOfDisks(): int;
  public function getReleaseDate(): \DateTime;
}

class CD implements Product
{
    // implmentacion
}

class DVD implements Product
{
    // implmentacion
}

Segregacion2.php

interface AgeAware
{
    public function getEdadMinima(): int;
}

class CD implements Product
{
    // implmentacion
}

class DVD implements Product, AgeAware
{
    // implmentacion
}