PHPSGI

PHP Server Gateway Interface specification

Abstract

This document specifies a standard interface between web servers and PHP web applications or frameworks. This interface is designed to promote web application portability and reduce the duplication of effort by web application framework developers.

Please keep in mind that PHPSGI is not Yet Another web application framework. PHPSGI is a specification to decouple web server environments from web application framework code. Nor is PHPSGI a web application API. Web application developers (end users) will not run their web applications directly using the PHPSGI interface, but instead are encouraged to use frameworks that support PHPSGI.

Implementation

Currently there is a PHPSGI Toolkit that supports SAPI backends (Apache mod_php and php-fpm) called Funk (https://github.com/phpsgi/Funk) you can check the toolkit implementation for more details.

To develop PHPSGI compatible applications, you can use composer to install phpsgi/phpsgi package.

composer require phpsgi/phpsgi "dev-master"

Status of This Specification

• Environment Variable Format
• Response Format
• Middleware Design (Request for comments)
• Application Design (Request for comments)
• Input Stream
• Output Stream
• Delayed Response
• File Upload Support
• Session and Cookie Support ($_SESSION. $_COOKIES).

Terminology

Web Servers

Web servers accept HTTP requests issued by web clients, dispatching those requests to web applications if configured to do so, and return HTTP responses to the request-initiating clients. Web servers can be any PHP SAPI web server (e.g. php-fpm, Apache PHP module ...) or any HTTP server implemented in PHP like "ReactPHP".

PHPSGI Server

A PHPSGI Server is a PHP program providing an environment for a PHPSGI application to run in.

PHPSGI specifying an interface for web applications and the main purpose of web applications being to be served to the Internet, a PHPSGI Server will most likely be either: part of a web server (like Apache module PHP), connected to a web server (with FastCGI), invoked by a web server (as in plain old CGI), or be a standalone web server itself, written entirely or partly in PHP.

There is, however, no requirement for a PHPSGI Server to actually be a web server or part of one, as PHPSGI only defines an interface between the server and the application, not between the server and the world.

A PHPSGI Server is often also called PHPSGI Application Container because it is similar to a Java Servlet container, which is Java process providing an environment for Java Servlets.

Applications

Web applications accept HTTP requests and return HTTP responses.

PHPSGI applications are web applications conforming to the PHPSGI interface, prescribing they take the form of a code reference with defined input and output.

For simplicity, PHPSGI Applications will also be referred to as Applications for the remainder of this document.

Middleware

Middleware is a PHPSGI application (a code reference) and a Server. Middleware looks like an application when called from a server, and it in turn can call other applications. It can be thought of a plugin to extend a PHPSGI application.

Framework developers

Framework developers are the authors of web application frameworks. They write adapters (or engines) which accept PHPSGI input, run a web application, and return a PHPSGI response to the server.

Web application developers

Web application developers are developers who write code on top of a web application framework. These developers should never have to deal with PHPSGI directly.

Specification

Application

A PHPSGI application is any callable variable, e.g. a Closure, an array contains callable payload or an object defined with __invoke magic method.

A PHPSGI application can be defined as follow:

$app = function(array & $environment, array $response) {
	// [response code,  response headers, body content ]
	return [ 200, [ 'Content-Type' => 'plain/text' ], 'Hello World' ];
};

The application can also be defined in PHP class:

use PHPSGI\App;

class MyApp implements App {
	public function call(array & $environment, array $response) {
		// [response code,  response headers, body content ]
		return [ 200, [ 'Content-Type' => 'plain/text' ], 'Hello World' ];
	}
	
	public function __invoke(array & $environment, array $response) {
		return $this->call($environment, $response);
	}
}

The Environment

The environment MUST be an associative array reference that includes CGI-like headers, as detailed below. The application is free to modify the environment. The environment MUST include these keys (adopted from PEP 333, Rack and JSGI) except when they would normally be empty.

When an environment key is described as a boolean, its value MUST conform to PHP's notion of boolean-ness. This means that an empty string or an explicit 0 are both valid false values. If a boolean key is not present, an application MAY treat this as a false value.

The environment array is derived from $_SERVER since it's supported by all current SAPI servers.

To create an environment array from PHP's super global arrays (for SAPI servers):

use Funk\Buffer\SAPIInputBuffer;

$environment = array_merge($_SERVER, [
	'parameters' => $_REQUEST,
	'body_parameters' => $_POST,
	'query_parameters' => $_GET,
    'phpsgi.input' => new SAPIInputBuffer();
]);

Then the environment array can be passed to the application closure:

$defaultResponse = [ 200, [], '' ];
$response = $app($environment, $defaultResponse);

The values for all CGI keys (named without a period) MUST be a scalar string.

See below for details.

• REQUEST_METHOD: The HTTP request method, such as "GET" or "POST". This MUST NOT be an empty string, and so is always required. SCRIPT_NAME: The initial portion of the request URL's path, corresponding to the application. This tells the application its virtual "location". This may be an empty string if the application corresponds to the server's root URI. If this key is not empty, it MUST start with a forward slash (/).

• PATH_INFO: The remainder of the request URL's path, designating the virtual "location" of the request's target within the application. This may be an empty string if the request URL targets the application root and does not have a trailing slash. This value should be URI decoded by servers in order to be compatible with RFC 3875. If this key is not empty, it MUST start with a forward slash (/).

• REQUEST_URI: The undecoded, raw request URL line. It is the raw URI path and query part that appears in the HTTP GET /... HTTP/1.x line and doesn't contain URI scheme and host names. Unlike PATH_INFO, this value SHOULD NOT be decoded by servers. It is an application's responsibility to properly decode paths in order to map URLs to application handlers if they choose to use this key instead of PATH_INFO.

• QUERY_STRING: The portion of the request URL that follows the ?, if any. This key MAY be empty, but MUST always be present, even if empty.

• SERVER_NAME, SERVER_PORT: When combined with SCRIPT_NAME and PATH_INFO, these keys can be used to complete the URL. Note, however, that HTTP_HOST, if present, should be used in preference to SERVER_NAME for reconstructing the request URL. SERVER_NAME and SERVER_PORT MUST NOT be empty strings, and are always required.

• SERVER_PROTOCOL: The version of the protocol the client used to send the request. Typically this will be something like "HTTP/1.0" or "HTTP/1.1" and may be used by the application to determine how to treat any HTTP request headers.

• CONTENT_LENGTH: The length of the content in bytes, as an integer. The presence or absence of this key should correspond to the presence or absence of HTTP Content-Length header in the request.

• CONTENT_TYPE: The request's MIME type, as specified by the client. The presence or absence of this key should correspond to the presence or absence of HTTP Content-Type header in the request.

A server should attempt to provide as many other CGI variables as are applicable. Note, however, that an application that uses any CGI variables other than the ones listed above are necessarily non-portable to web servers that do not support the relevant extensions.

• phpsgi.version: An array reference [1,1] representing this version of PHPSGI. The first number is the major version and the second it the minor version.

• phpsgi.input: the input stream. See below for details.

• phpsgi.nonblocking: A boolean which is true if the server is calling the application in an non-blocking event loop.

• phpsgi.streaming: A boolean which is true if the server supports callback style delayed response and streaming writer object.

The Input Stream

The input stream in phpsgi.input is an resource object which streams the raw HTTP POST or PUT data. The file handle MUST be opened in binary mode.

The input stream object MUST respond to read and MAY implement seek. Here is an example that implements a stream Buffer object for php://input:

use PHPSGI\Buffer\ReadableBuffer;

class SAPIInputBuffer implements ReadableBuffer
{
    protected $fd;

    public function __construct()
    {
        $this->fd = fopen('php://input', 'r');
    }

    public function read($bytes)
    {
        return fread($this->fd, $bytes);
    }

    public function __destruct()
    {
        fclose($this->fd);
    }
}

If your Buffer supports seek, you can implements your buffer class with interface PHPSGI\Buffer\SeekableBuffer.

The minimal requirement also conforms to the implementation of event extension's EventBuffer http://php.net/manual/en/eventbuffer.read.php.

And please note that php://input doesn't support seek. see http://php.net/manual/en/wrappers.php.php#wrappers.php.input for more details.

Note: Prior to PHP 5.6, a stream opened with php://input could only be read once; the stream did not support seek operations. However, depending on the SAPI implementation, it may be possible to open another php://input stream and restart reading. This is only possible if the request body data has been saved. Typically, this is the case for POST requests, but not other request methods, such as PUT or PROPFIND.

Middleware

package phpsgi/phpsgi provides out-of-box middleware base class and app interface (implemented in both extension and pure php), you can define your middlewares like this:

use PHPSGI\Middleware;

class MyMiddleware extends Middleware
{
	public function call(array & $environment, array $response) {
		// preprocessing ...
		
		// call the next middleware or application
		$response = parent::call($environment, $response);
		
		// postprocessing 
		
		return $response;
	}
}

To wrap your application with middlewares:

$app = new MyMiddleware($app);
$app = new XHProfMiddleware($app);
$response = $app($environment, $response);

Questions and Answers

1. Why must environment be an associative array? What's wrong with using a class?

   The rationale for requiring an associative array is to maximize portability between servers. In practice, however, most servers will probably find a Request class adequate to their needs, and thus framework authors will come to expect the full set of features to be available, since they will be there more often than not. But, if some server chooses not to use an associative array, then there will be interoperability problems despite that server's "conformance" to spec. Therefore, making a an associative array mandatory simplifies the specification and guarantees interoperabilty.

2. Why is this interface so low-level? I want feature X! (e.g. cookies, sessions, persistence, ...)

   This isn't Yet Another PHP Web Framework. It's just a way for frameworks to talk to web servers, and vice versa. If you want these features, you need to pick a web framework that provides the features you want. And if that framework lets you create a PHPSGI application, you should be able to run it in most PHPSGI-supporting servers. Also, some PHPSGI servers may offer additional services via objects provided in their environ dictionary; see the applicable server documentation for details. (Of course, applications that use such extensions will not be portable to other PHPSGI-based servers.)

3. Why use CGI variables instead of good old HTTP headers? And why mix them in with PHPSGI-defined variables?

   Many existing web frameworks are built heavily upon the CGI spec, especially the global $_SERVER variable, and existing SAPI web servers know how to generate CGI variables. In contrast, alternative ways of representing inbound HTTP information are fragmented and lack market share. Thus, using the CGI "standard" seems like a good way to leverage existing implementations. As for mixing them with PHPSGI variables, separating them would just require two assoc array arguments to be passed around, while providing no real benefits.

Changelogs

Acknowledgements

Some parts of this specification are adopted from the following specifications.

• PSGI http://search.cpan.org/~miyagawa/PSGI-1.102/PSGI.pod
• PEP333 Python Web Server Gateway Interface http://www.python.org/dev/peps/pep-0333
• Rack http://rack.rubyforge.org/doc/SPEC.html
• JSGI Specification http://jackjs.org/jsgi-spec.html

I'd like to thank authors of these great documents.

Author

Yo-An Lin yoanlin93@gmail.com

Copyright and Licenese

Copyright Yo-An Lin, 2015-present

This document is licensed under the Creative Commons license by-sa.