Example: Use the Builder pattern to build immutable objects (usage see below in example.php)

example.php

use Example\Http\Request;
use Phly\Http\Stream;
use Phly\Http\Uri;

$requestBuilder = Request::builder(new Uri('http://example.com/'), new Stream(fopen('php://memory')));

$requestBuilder->setMethod('POST')->setTarget('/ping')
$requestBuilder->getHeaders()->add([
  'Accept'     => 'text/html',
  'User-Agent' => 'Example-Robot'
  ]);

// parameters of $requestBuilder may be changed until request is built ...

$request = new Request($requestBuilder);

// $request is now an immutable Request instance that can be passed to a middleware

Request.php

<?php
namespace Example\Http;

use Psr\Http\Message\UriInterface;
use Psr\Http\Message\StreamInterface;
use Symfony\Component\HttpFoundation\HeaderBag;

/**
 * Immutable request object
 */
final class Request
{
    /**
     * @var StreamInterface
     */
    private $body;
    /**
     * @var UriInterface
     */
    private $uri;
    /**
     * @var string[]
     */
    private $headers;
    /**
     * @var string
     */
    private $method;
    /**
     * @var string
     */
    private $target;

    public static function builder(UriInterface $uri, StreamInterface $body)
    {
        return new RequestBuilder($uri, $body);
    }

    public function __construct(RequestBuilder $builder)
    {
        $this->body = $builder->getBody();
        $this->uri = $builder->getUri();
        $this->headers = $builder->getHeaders()->all();
        $this->method = $builder->getMethod();
        $this->target = $builder->getTarget();
    }

    /**
     * @return StreamInterface
     */
    public function getBody()
    {
        return $this->body;
    }

    /**
     * @return UriInterface
     */
    public function getUri()
    {
        return $this->uri;
    }

    /**
     * @return \string[]
     */
    public function getHeaders()
    {
        return $this->headers;
    }

    /**
     * @return string
     */
    public function getMethod()
    {
        return $this->method;
    }

    /**
     * @return string
     */
    public function getTarget()
    {
        return $this->target;
    }
}

class RequestBuilder
{
    /**
     * @var StreamInterface
     */
    private $body;
    /**
     * @var UriInterface
     */
    private $uri;
    /**
     * @var HeaderBag
     */
    private $headers;
    /**
     * @var string
     */
    private $method;
    /**
     * @var string
     */
    private $target;

    public function __construct(UriInterface $uri, StreamInterface $body)
    {
        // required values:
        $this->uri = $uri;
        $this->body = $body;

        // default values:
        $this->headers= new HeaderBag();
        $this->method = 'GET';
        $this->target = '/';
    }

    /**
     * @return StreamInterface
     */
    public function getBody()
    {
        return $this->body;
    }

    /**
     * @param StreamInterface $body
     */
    public function setBody($body)
    {
        $this->body = $body;
    }

    /**
     * @return UriInterface
     */
    public function getUri()
    {
        return $this->uri;
    }

    /**
     * @param UriInterface $uri
     */
    public function setUri($uri)
    {
        $this->uri = $uri;
    }

    /**
     * @return string[]
     */
    public function getHeaders()
    {
        return $this->headers;
    }

    /**
     * @return string
     */
    public function getMethod()
    {
        return $this->method;
    }

    /**
     * @param string $method
     */
    public function setMethod($method)
    {
        $this->method = $method;
    }

    /**
     * @return string
     */
    public function getTarget()
    {
        return $this->target;
    }

    /**
     * @param string $target
     */
    public function setTarget($target)
    {
        $this->target = $target;
    }

}