CI_PSR7.php

<?php

$app = new Zend\Stratigility\MiddlewarePipe();

// Direct instantiation, with a callback handler, request, and response
$server = Zend\Diactoros\Server::createServer($app, $_SERVER, $_GET, $_POST, $_COOKIE, $_FILES);
$app->pipe('/hello', function ($req, $res, $next) {
	return $res->getBody()->write('Hello World!');
});
$app->pipe(function (Psr\Http\Message\RequestInterface $req, Psr\Http\Message\ResponseInterface $res, $next) {
	$app = new Slim\App();
	$app->get('/hi', function ($req, $res) {
		return $res->getBody()->write("I am in Slim");
	});
	/** @var Zend\Diactoros\Response $response */
	$response = $app($req, $res);
	if ($response->getStatusCode() !== 404) {
		return $response;
	}
	return $next($req, $res);
});
$app->pipe(new App\Core\CodeigniterPSR7Middleware());

$server->listen();

CodeigniterInit.php

<?php


class CodeigniterInit
{
	public static function run()
	{
		/*
		 *---------------------------------------------------------------
		 * SYSTEM FOLDER NAME
		 *---------------------------------------------------------------
		 *
		 * This variable must contain the name of your "system" folder.
		 * Include the path if the folder is not in the same directory
		 * as this file.
		 */
		$system_path = 'system';

		/*
         *---------------------------------------------------------------
         * APPLICATION FOLDER NAME
         *---------------------------------------------------------------
         *
         * If you want this front controller to use a different "application"
         * folder than the default one you can set its name here. The folder
         * can also be renamed or relocated anywhere on your server. If
         * you do, use a full server path. For more info please see the user guide:
         * http://codeigniter.com/user_guide/general/managing_apps.html
         *
         * NO TRAILING SLASH!
         */
		$application_folder = 'application';

		/*
         *---------------------------------------------------------------
         * VIEW FOLDER NAME
         *---------------------------------------------------------------
         *
         * If you want to move the view folder out of the application
         * folder set the path to the folder here. The folder can be renamed
         * and relocated anywhere on your server. If blank, it will default
         * to the standard location inside your application folder. If you
         * do move this, use the full server path to this folder.
         *
         * NO TRAILING SLASH!
         */
		$view_folder = '';

		/*
         * ---------------------------------------------------------------
         *  Resolve the system path for increased reliability
         * ---------------------------------------------------------------
         */

		// Set the current directory correctly for CLI requests
		if (defined('STDIN'))
		{
			chdir(dirname(__FILE__));
		}

		if (($_temp = realpath($system_path)) !== FALSE)
		{
			$system_path = $_temp.'/';
		}
		else
		{
			// Ensure there's a trailing slash
			$system_path = rtrim($system_path, '/').'/';
		}

		// Is the system path correct?
		if ( ! is_dir($system_path))
		{
			header('HTTP/1.1 503 Service Unavailable.', TRUE, 503);
			echo 'Your system folder path does not appear to be set correctly. Please open the following file and correct this: '.pathinfo(__FILE__, PATHINFO_BASENAME);
			exit(3); // EXIT_CONFIG
		}

		/*
         * -------------------------------------------------------------------
         *  Now that we know the path, set the main path constants
         * -------------------------------------------------------------------
         */
		// The name of THIS file
		define('SELF', pathinfo(__FILE__, PATHINFO_BASENAME));

		// Path to the system folder
		define('BASEPATH', str_replace('\\', '/', $system_path));

		// Path to the front controller (this file)
		define('FCPATH', str_replace(SELF, '', __FILE__));

		// Name of the "system folder"
		define('SYSDIR', trim(strrchr(trim(BASEPATH, '/'), '/'), '/'));

		// The path to the "application" folder
		if (is_dir($application_folder))
		{
			if (($_temp = realpath($application_folder)) !== FALSE)
			{
				$application_folder = $_temp;
			}

			define('APPPATH', $application_folder.DIRECTORY_SEPARATOR);
		}
		else
		{
			if ( ! is_dir(BASEPATH.$application_folder.DIRECTORY_SEPARATOR))
			{
				header('HTTP/1.1 503 Service Unavailable.', TRUE, 503);
				echo 'Your application folder path does not appear to be set correctly. Please open the following file and correct this: '.SELF;
				exit(3); // EXIT_CONFIG
			}

			define('APPPATH', BASEPATH.$application_folder.DIRECTORY_SEPARATOR);
		}

		// The path to the "views" folder
		if ( ! is_dir($view_folder))
		{
			if ( ! empty($view_folder) && is_dir(APPPATH.$view_folder.DIRECTORY_SEPARATOR))
			{
				$view_folder = APPPATH.$view_folder;
			}
			elseif ( ! is_dir(APPPATH.'views'.DIRECTORY_SEPARATOR))
			{
				header('HTTP/1.1 503 Service Unavailable.', TRUE, 503);
				echo 'Your view folder path does not appear to be set correctly. Please open the following file and correct this: '.SELF;
				exit(3); // EXIT_CONFIG
			}
			else
			{
				$view_folder = APPPATH.'views';
			}
		}

		if (($_temp = realpath($view_folder)) !== FALSE)
		{
			$view_folder = $_temp.DIRECTORY_SEPARATOR;
		}
		else
		{
			$view_folder = rtrim($view_folder, '/\\').DIRECTORY_SEPARATOR;
		}

		define('VIEWPATH', $view_folder);

		/*
         * --------------------------------------------------------------------
         * LOAD THE BOOTSTRAP FILE
         * --------------------------------------------------------------------
         *
         * And away we go...
         */
		require_once BASEPATH.'core/CodeIgniter.php';
	}
}

CodeigniterPSR7Middleware.php

<?php

namespace App\Core;

use Psr\Http\Message\ResponseInterface as Response;
use Psr\Http\Message\ServerRequestInterface as Request;
use Zend\Stratigility\MiddlewareInterface;

class CodeigniterPSR7Middleware implements MiddlewareInterface
{
	/**
	 * Process an incoming request and/or response.
	 *
	 * Accepts a server-side request and a response instance, and does
	 * something with them.
	 *
	 * If the response is not complete and/or further processing would not
	 * interfere with the work done in the middleware, or if the middleware
	 * wants to delegate to another process, it can use the `$out` callable
	 * if present.
	 *
	 * If the middleware does not return a value, execution of the current
	 * request is considered complete, and the response instance provided will
	 * be considered the response to return.
	 *
	 * Alternately, the middleware may return a response instance.
	 *
	 * Often, middleware will `return $out();`, with the assumption that a
	 * later middleware will return a response.
	 *
	 * @param Request $request
	 * @param Response $response
	 * @param null|callable $out
	 * @return null|Response
	 */
	public function __invoke(Request $req, Response $res, callable $out = null)
	{
		ob_start();

		\CodeigniterInit::run();

		$headers = headers_list();
		$statusCode = http_response_code();
		header_remove();

		$response = $res->withStatus($statusCode);

		foreach ($headers as $header) {
			$pieces = explode(':', $header);
			$headerName = array_shift($pieces);
			$response = clone $response->withAddedHeader($headerName, trim(implode(':', $pieces)));
		}

		return $response->write(ob_get_clean());
	}
}