Compiling PHP to WebAssembly and using it in JavaScript

The src/php-wasm module brings PHP into JavaScript as a WebAssembly module:

import { createPHP } from 'php-wasm';

const PHPLoaderModule = await import('/php.js');
const php = await createPHP(PHPLoaderModule);
console.log(`<?php echo "Hello from PHP!";`).stdout);
// Output: "Hello from PHP!"

It consists of two major building blocks:

See also the API Reference.

PHP to WebAssembly build pipeline

The pipeline lives in wasm/Dockerfile. It was originally forked from seanmorris/php-wasm

In broad strokes, that Dockerfile:

  • Installs all the necessary linux packages (like build-essential)
  • Downloads PHP and the required libraries, e.g. sqlite3.
  • Applies a few patches.
  • Compiles everything using Emscripten, a drop-in replacement for the C compiler.
  • Compiles php_wasm.c – a convenient API for JavaScript.
  • Outputs a php.wasm file and one or more JavaScript loaders, depending on the configuration.
  • Transforms the Emscripten's default php.js output into an ESM module with additional features.

To find out more about each step, refer directly to the Dockerfile.


To build all PHP versions, run npm run build:php:web in the repository root. You'll find the output files in src/php-wasm/build-wasm. To build a specific version, run npm run build:php:web:<VERSION>, e.g. npm run build:php:web:8.2.

PHP extensions

PHP is built with several extensions listed in the Dockerfile.

Some extensions, like zip, can be turned on or off during the build. Others, like sqlite3, are hardcoded.

If you need to turn off one of the hardcoded extensions, feel free to open an issue in this repo. Better yet, this project needs contributors. You are more than welcome to open a PR and author the change you need.

C API exposed to JavaScript

The C API exposed to JavaScript lives in the wasm/build-assets/php_wasm.c file. The most important functions are:

  • void phpwasm_init() – It creates a new PHP context and must be called before running any PHP code.
  • int phpwasm_run(char *code) – Runs a PHP script and writes the output to /tmp/stdout and /tmp/stderr. Returns the exit code.
  • void phpwasm_refresh() – Destroy the current PHP context and starts a new one. Call it after running one PHP script and before running another.

Refer to the inline documentation in php_wasm.c to learn more.

Build configuration

The build is configurable via the Docker --build-arg feature. Currently it's not possible to specify these options via npm run build:php:web so you'll need to run docker build manually. For example, to change the PHP version, specify the PHP_VERSION option during the build:

docker build . --build-arg PHP_VERSION=7.4.0

Supported build options:

  • PHP_VERSION – The PHP version to build, default: 8.0.24. This value must point to an existing branch of the repository when prefixed with PHP-. For example, 7.4.0 is valid because the branch PHP-7.4.0 exists, but just 7 is invalid because there's no branch PHP-7. The PHP versions that are known to work are 7.4.* and 8.0.*. Others likely work as well but they haven't been tried.
  • EMSCRIPTEN_ENVIRONMENTweb or node, default: web. The platform to build for. When building for web, two JavaScript loaders will be created: php-web.js and php-webworker.js. When building for Node.js, only one loader called php-node.js will be created.
  • WITH_LIBXMLyes or no, default: no. Whether to build with libxml2 and the dom, xml, and simplexml PHP extensions (DOMDocument, SimpleXML, ..).
  • WITH_LIBZIPyes or no, default: yes. Whether to build with zlib, libzip, and the zip PHP extension (ZipArchive).
  • WITH_VRZNOyes or no, default: yes when PHP_VERSION is 7.*. Whether to build with the vrzno PHP extension that enables running JavaScript code from PHP.
  • WITH_NODEFSyes or no, default: no. Whether to include the Emscripten's NODEFS JavaScript library. It's useful for loading files and mounting directories from the local filesystem when running php.wasm from Node.js.

JavaScript module

The php.js file generated by this build process is not a vanilla Emscripten module. Instead, it's an ESM module that wraps the regular Emscripten output and adds some extra functionality.

Here's the API it exposes:

// php.wasm size in bytes:
export const dependenciesTotalSize = 5644199;

// php.wasm filename:
export const dependencyFilename = 'php.wasm';

// Run Emscripten's generated module:
export default function (jsEnv, emscriptenModuleArgs) {}

JavaScript bindings for the WebAssembly PHP

php-wasm provides a JavaScript API to interact with the WebAssembly module. It lives in the src directory and consists of:

  • A PHP class to directly interface with the WebAssembly module.
  • A PHPServer class to use PHP for handling HTTP requests.
  • A PHPBrowser class to handle cookies and redirects emitted by PHPServer


To build the JavaScript API, run npm run build:js in the repository root.


Below you'll find a few especially relevant parts of the API. Consult the php-wasm API reference page to learn about the rest of it.


   phpLoaderModule: any,
   runtime: JavascriptRuntime,
   phpModuleArgs?: any,
   dataDependenciesModules?: any[]
): Promise<PHP>

  • phpLoaderModule – The ESM-wrapped Emscripten module. Consult the Dockerfile for the build process.
  • runtime – The current JavaScript environment. One of: NODE, WEB, or WEBWORKER.
  • phpModuleArgs – Optional. The Emscripten module arguments, see
  • dataDependenciesModules – Optional. A list of the ESM-wrapped Emscripten data dependency modules.
  • Returns: PHP instance.

Initializes the PHP runtime with the given arguments and data dependencies.

This function handles the entire PHP initialization pipeline. In particular, it:

  • Instantiates the Emscripten PHP module
  • Wires it together with the data dependencies and loads them
  • Ensures is all happens in a correct order
  • Waits until the entire loading sequence is finished

Basic usage:

 const phpLoaderModule = await import("/php.js");
 const php = await startPHP(phpLoaderModule, "web");
 console.log(`<?php echo "Hello, world!"; `));
 // { stdout: ArrayBuffer containing the string "Hello, world!", stderr: [''], exitCode: 0 }

The /php.js module:

In the basic usage example, php.js is not a vanilla Emscripten module. Instead, it's an ESM module that wraps the regular Emscripten output and adds some extra functionality. It's generated by the Dockerfile shipped with this repo. Here's the API it provides:

// php.wasm size in bytes:
export const dependenciesTotalSize = 5644199;

// php.wasm filename:
export const dependencyFilename = 'php.wasm';

// Run Emscripten's generated module:
export default function(jsEnv, emscriptenModuleArgs) {}

PHP Filesystem:

Once initialized, the PHP has its own filesystem separate from the project files. It's provided by Emscripten and uses its FS library.

The API exposed to you via the PHP class is succinct and abstracts await certain unintuitive parts of low-level FS interactions.

Here's how to use it:

// Recursively create a /var/www directory

// false

php.writeFile('/var/www/file.txt', 'Hello from the filesystem!');

// true

// "Hello from the filesystem!

// Delete the file:

For more details consult the PHP class directly.

Data dependencies:

Using existing PHP packages by manually recreating them file-by-file would be quite inconvenient. Fortunately, Emscripten provides a "data dependencies" feature.

Data dependencies consist of a file and a dependency.js loader and can be packaged with the tool. This project requires wrapping the Emscripten-generated dependency.js file in an ES module as follows:

  1. Prepend export default function(emscriptenPHPModule) {';
  2. Prepend export const dependencyFilename = '<DATA FILE NAME>';
  3. Prepend export const dependenciesTotalSize = <DATA FILE SIZE>;
  4. Append }

Be sure to use the --export-name="emscriptenPHPModule" option.

You want the final output to look as follows:

export const dependenciesTotalSize = 5644199;
export const dependencyFilename = '';
export default function(emscriptenPHPModule) {
   // Emscripten-generated code:
   var Module = typeof emscriptenPHPModule !== 'undefined' ? emscriptenPHPModule : {};
   // ... the rest of it ...

Such a constructions enables loading the dependency.js as an ES Module using import("/dependency.js").

Once it's ready, you can load PHP and your data dependencies as follows:

 const [phpLoaderModule, wordPressLoaderModule] = await Promise.all([
 const php = await startPHP(phpLoaderModule, "web", {}, [wordPressLoaderModule]);



class PHPServer 

A fake PHP server that handles HTTP requests but does not bind to any port.


PHPServer(php: PHP, config: PHPServerConfigation)

  • php – The PHP instance.
  • config – Server configuration.

Constructs a new instance of the PHPServer class


  • absoluteUrl readonly string – The absolute URL of this PHPServer instance.
  • php PHP – The PHP instance


request(request: PHPRequest): Promise<PHPResponse>

  • request – The request.
  • Returns: The response.

Serves the request – either by serving a static file, or by dispatching it to the PHP runtime.


import { createPHP, PHPServer } from 'php-wasm';

const PHPLoaderModule = await import('/php.js');
const php = await createPHP(PHPLoaderModule);

// Create a file to serve:
php.writeFile('/www/index.php', '<?php echo "Hi from PHP!"; ');

// Create a server instance:
const server = new PHPServer(php, {
    // PHP FS path to serve the files from:
    documentRoot: '/www',

    // Used to populate $_SERVER['SERVER_NAME'] etc.:
    absoluteUrl: ''

   server.request({ path: '/index.php' }).body
// Output: "Hi from PHP!"