# Introduction
Phasis is a pure-PHP JavaScript engine. It lexes, parses, and executes ECMAScript in the host PHP process — without `exec('node …')`, without FFI, and without a custom binary extension. It requires three standard PHP extensions: `ext-mbstring`, `ext-bcmath` and `ext-curl` (the default `fetch()` transport). `ext-intl` and `ext-openssl` are optional. See [Getting Started](/docs/getting-started) for builds that leave `bcmath` out.

- **Embeddable** — drop a JS runtime into any PHP app, framework, or CLI tool via the `Phasis\Engine` class.
- **Spec-compliant** — passes the official test262 suite at 100 %, every category, no skips.
- **Web APIs** — full **Web Platform Pack** (URL, TextEncoder/Decoder, atob/btoa, structuredClone, performance, DOMException), full **Fetch Pack** (fetch, Request, Response, Headers, Body, AbortController/Signal, Blob, File, FormData, EventTarget, full WHATWG Streams, navigator), plus `crypto` + `SubtleCrypto`, `WebSocket`, `XMLHttpRequest`, and a real event loop. 100 % of imported Web Platform Tests pass.
- **Modern surface** — arrow functions, classes, async/await, Promises, generators, Symbol, BigInt, Proxy, Reflect, Temporal, Intl, TypedArrays, full module support.
- **PHP↔JS interop** — pass PHP scalars and arrays into JS (arrays are copied), bind PHP closures as JS functions, and call JS functions from PHP. PHP objects cannot be shared with JS yet; see [Shared objects](/docs/interop/shared-objects).

## Why Phasis exists

PHP applications that need to run user-supplied JavaScript — templating engines, frontend SSR shims, data-validation rules, content sandboxes, headless test runners — usually shell out to Node.js or skip the feature. Either path adds operational complexity: a second runtime, a serialization boundary, a network of subprocess pipes, and a hostile deployment story for shared hosting.

Phasis closes that gap. Your PHP code can lex, parse, and execute JavaScript using only PHP 8.2+ with `mbstring`, `bcmath` and `curl`. The engine ships as a Composer package and exposes a single small API surface.

## What it does

- Implements the full ECMAScript standard library: `Array`, `String`, `Object`, `Math`, `JSON`, `Date`, `RegExp`, `Map`, `Set`, `Promise`, `Proxy`, `Reflect`, `Symbol`, `BigInt`, `TypedArray`, `Temporal`, `Intl`, generators, async iterators, modules.
- Ships the **Web Platform** APIs that real-world JS libraries pre-check for: `URL`, `URLSearchParams`, `TextEncoder`/`TextDecoder`, `atob`/`btoa`, `structuredClone`, `performance`, `DOMException`. See [Web APIs](/docs/compatibility/web-apis) for the full surface.
- Ships **`fetch`** with the full Fetch family — `Request`, `Response`, `Headers`, body consumers, `AbortController`/`AbortSignal`, `Blob`, `File`, `FormData`, `EventTarget`/`Event`, and full WHATWG Streams (`ReadableStream`, `WritableStream`, `TransformStream`, BYOB, queuing strategies). Real HTTP works out of the box via PHP's `ext-curl`.
- Ships **`crypto`** + full `SubtleCrypto` (SHA family, HMAC, AES-GCM/CBC/CTR, RSA-OAEP/PSS/PKCS1, ECDSA, ECDH, HKDF, PBKDF2), **`WebSocket`** (RFC 6455 + swappable transport), and **`XMLHttpRequest`** (layered over the same fetch transport).
- Ships a real **event loop**: `setTimeout`, `setInterval`, `clearTimeout`, `clearInterval`, `queueMicrotask`, plus Stage-3 `AsyncContext` for value propagation across async boundaries.
- Supports modern syntax — destructuring, spread/rest, computed properties, optional chaining, nullish coalescing, template literals, tagged templates, classes (with private fields, static blocks, decorators), top-level await.
- Converts values across the PHP↔JS boundary. Scalars and arrays are copied in, PHP callables become JS functions, and JS results come back as PHP scalars and arrays. See [Value conversion](/docs/interop/values) for the exact rules.
- Runs under host control. The host owns the `fetch` transport and policy hooks, the event loop runs on virtual time, and a fixed call-stack ceiling stops runaway recursion. A loop-iteration limit exists, but it is not yet enforced on loops the bytecode VM runs, and there are no string-length, output or wall-clock limits; see [API: setLimit](/docs/api#setlimit).
- Is verified against Node.js (V8). CI runs the unit and oracle suites on every push to `main` and every pull request, and the Web Platform Tests and popular-package suites when the engine source changes; the full test262 matrix is run by hand and its snapshot committed to `COMPAT.md`. See [Compatibility](/docs/compatibility).

## What it is not

Phasis is not a JIT compiler. It is a tree-walking interpreter with an opportunistic bytecode VM. Expect ~100× the runtime of V8 on dispatch-bound workloads. The trade-off is zero dependencies, pure PHP, and host-controlled execution — perfect for embedding JavaScript into PHP apps where correctness and isolation matter more than raw throughput.

## Where to start

- [Getting Started](/docs/getting-started) — install and run your first script.
- [CLI](/docs/cli) — `bin/phasis` for running files, evaluating source, and dumping the AST.
- [API](/docs/api) — embed `Phasis\Engine` in your PHP app.
- [Interop](/docs/interop) — pass PHP values and callables to JavaScript.
- [Compatibility](/docs/compatibility) — test262 coverage and supported features.
- [Parser](/docs/parser) — parse JavaScript into a typed AST or ESTree without executing it.
- [Formatter](/docs/formatter) — format JavaScript, JSON and CSS with prettier-compatible layout.
- [Advanced](/docs/advanced) — architecture, bytecode VM, benchmarks, and the oracle testing model.