Compile PHP Wasm

Compiling PHP.wasm

Patterns for compiling PHP.wasm main modules and dynamic extension side modules in the WordPress Playground repository.

Requires: Docker, Node.js (version from .nvmrc), npm

Quick Reference

```bash
# Recompile a specific version for ALL platforms and modes (web+node, jspi+asyncify)
npx nx recompile-php:all php-wasm-web -- --PHP_VERSION=8.5
npx nx recompile-php:all php-wasm-node -- --PHP_VERSION=8.5

# Recompile a specific version + platform + single mode
npx nx recompile-php:jspi php-wasm-web -- --PHP_VERSION=8.5
npx nx recompile-php:asyncify php-wasm-web -- --PHP_VERSION=8.5
npx nx recompile-php:jspi php-wasm-node -- --PHP_VERSION=8.5
npx nx recompile-php:asyncify php-wasm-node -- --PHP_VERSION=8.5

# Recompile all PHP versions for a platform
npm run recompile:php:web
npm run recompile:php:node

# Debug build (DWARF info)
# Use when you need to step through WASM in a debugger (Chrome DevTools DWARF
# support) or need better stack traces with C function names. Helpful for
# crashes like `RuntimeError: unreachable` where you need to identify which
# C function is involved. Produces much larger binaries.
npx nx recompile-php:all php-wasm-web -- --WITH_DEBUG=yes
npx nx recompile-php:all php-wasm-node -- --WITH_DEBUG=yes

# Source maps
# Use when you want to map JS glue code back to its Emscripten-generated
# source locations. Lighter weight than DWARF but only covers the JS side,
# not the WASM internals.
npx nx recompile-php:all php-wasm-web -- --WITH_SOURCEMAPS=yes
npx nx recompile-php:all php-wasm-node -- --WITH_SOURCEMAPS=yes

# Reset caches before rebuilding
node node_modules/.bin/nx reset
docker rmi php-wasm:latest

## Build Pipeline Overview

The build system lives in `packages/php-wasm/compile/`. The pipeline is:

Dockerfile (Emscripten + PHP source + patches) ↓ docker build .wasm binary + .js glue file ↓ post-link Dockerfile patches (replace.sh) Patched .js glue file ↓ NX executor copies to dist/ Final artifacts in package dist/

Key files:

| File | Purpose |
|------|---------|
| `Dockerfile` | Main build — downloads PHP source, applies patches, runs `emcc` |
| `Makefile` | Orchestrates Docker builds per PHP version |
| `build.js` | Node script invoked by NX executors |
| `.emcc-php-wasm-flags` | Emscripten linker flags (generated during build) |
| `.emcc-php-wasm-sources` | Source/library paths for linking (generated during build) |
| `replace.sh` / Dockerfile `RUN sed` | Post-link patches to the compiled JS glue file |

## Main Module Compilation

### Emscripten flags that matter

| Flag | Purpose |
|------|---------|
| `ASYNCIFY` | Enables Asyncify (unwind/rewind for async JS calls) |
| `ASYNCIFY_ONLY=[func1,func2,...]` | Whitelist of functions Asyncify instruments (critical for binary size) |
| `JSPI` | Enables JSPI (V8 native stack switching, replaces Asyncify) |
| `JSPI_IMPORTS=[func1,...]` | JS imports wrapped with `WebAssembly.Suspending` |
| `JSPI_EXPORTS=[func1,...]` | WASM exports wrapped with `WebAssembly.promising` |
| `MAIN_MODULE=1` | Enables `dlopen` — exports all symbols for dynamic linking |
| `MAIN_MODULE=2` | Like `=1` but only exports explicitly listed symbols |
| `EXPORTED_FUNCTIONS=[...]` | C functions accessible from JS |
| `EXPORTED_RUNTIME_METHODS=[...]` | Emscripten runtime helpers accessible from JS |
| `ENVIRONMENT=web,worker` | Target environments (affects code generation) |
| `INITIAL_MEMORY=256MB` | Starting linear memory size |

### MAIN_MODULE for dynamic extensions

When adding `dlopen` support (`MAIN_MODULE=1`):

- **Never use `-l` flags for C libraries.** Library directories contain both
  `.a` (static) and `.so` (WASM side module) files. With `MAIN_MODULE`, PIC
  mode makes the linker prefer `.so`. When `wasm-ld` encounters a `.so`
  under `--whole-archive`, it crashes with SIGSEGV. Fix: use explicit `.a`
  paths in `.emcc-php-wasm-sources` instead of `-l` flags.

- **`MAIN_MODULE=1` has linker limitations.** `wasm-ld` cannot handle
  `--whole-archive` + `--experimental-pic` on archives as large as
  `libphp.a`. This is a fundamental limitation. Start with `=1` (exports
  all symbols), then consider `=2` for optimization.

- **ENVIRONMENT=web,worker changes code generation.** With a single
  environment, Emscripten hardcodes booleans (`ENVIRONMENT_IS_WEB = true`).
  With multiple, it generates runtime detection. Post-processing regex
  patterns in the Dockerfile must handle both forms.

### Post-link Dockerfile patches

The Dockerfile uses `sed` / `replace.sh` to modify the compiled JS glue
file after Emscripten runs. Common patches:

- **Inject `_malloc` binding** when it's not auto-exposed:
  ```js
  PHPLoader['malloc'] = wasmExports['malloc'];
  // Injected right after assignWasmExports() in the glue file

• Cache Asyncify buffers to prevent memory.grow() corruption during handleSleep() (see debug-php-wasm-main-module skill for details)
• Guard ENVIRONMENT_IS_* substitution for multi-environment builds

Emscripten version upgrade checklist

When upgrading Emscripten, expect these categories of breakage:

1. Removed/renamed APIs:

   • setErrNo() removed — use HEAP32[___errno_location() >> 2] = code
   • _malloc/_free no longer auto-exposed — add to EXPORTED_FUNCTIONS
   • HEAPU8/HEAPU32 need explicit EXPORTED_RUNTIME_METHODS

2. Stricter Clang compiler:

   • -Wincompatible-pointer-types becomes an error. Fix: correct the types, not the warning level.
   • PHP version checks like #if PHP_MAJOR_VERSION >= 8 may be too broad (e.g. zend_file_handle.filename is const char * in 8.0 but zend_string * in 8.1+)

3. JSPI behavioral changes:

   • WASI syscall wrappers (e.g. fd_close) may gain JS intermediate frames, breaking JSPI suspension. Symptom: startup hangs silently. Fix: remove from JSPI_IMPORTS/JSPI_EXPORTS.
   • exitRuntime() → __funcs_on_exit() may trigger JSPI suspension. Add to JSPI_EXPORTS and EXPORTED_FUNCTIONS.

4. Debugging approach:

   • Build PHP 8.4 first (fewest compatibility issues)
   • Fix build errors, then run JSPI tests
   • Once 8.4 passes, test 8.0 and 7.4 for version-specific issues
   • Patch PHP source files (php*.patch) for older versions as needed

Side Module Compilation

Side modules are PHP extensions (Xdebug, intl, GD, etc.) compiled as WASM shared libraries loaded via dlopen.

Build requirements

The extension build needs a minimal PHP installation (for phpize and headers). Key Emscripten-specific requirements:

Asyncify side modules

When the side module uses custom renamed imports (e.g. -Drecv=wasm_recv), you MUST pass -sASYNCIFY_IMPORTS=<custom_name>:

export EMCC_FLAGS="-sSIDE_MODULE -sASYNCIFY -sASYNCIFY_IMPORTS=wasm_recv"

Without this, Binaryen won't instrument call sites for those imports — locals won't be saved/restored, causing table index is out of bounds during Asyncify rewind.

Libtool issues

Libtool refuses to create WASM shared libraries. Two workarounds:

1. Patch libtool's archive_cmds — replace $CC with emcc $EMCC_FLAGS -shared --whole-archive <static archives> --no-whole-archive
2. Bypass libtool entirely — manually link with em++, discovering all .o files recursively (find . -path '*/.libs/*.o')

When using approach 2, check subdirectories — C++ libraries often produce .o files in nested paths that build scripts miss.

Pre-compiled artifacts

ICU .a archives and other pre-built artifacts committed to the repo may not match current build flags. When MAIN_MODULE=1 requires PIC, pre-built non-PIC archives cause R_WASM_MEMORY_ADDR_SLEB relocation errors. Rebuild from source if flags changed.

Cache Busting

Docker BuildKit caches aggressively. Before rebuilding:

# Remove the image to force a true rebuild
docker rmi php-wasm:latest

# docker builder prune alone is NOT sufficient — BuildKit reuses
# intermediate layers from existing images

# Also reset NX cache
node node_modules/.bin/nx reset

WASM Binary Inspection

When the build produces .wasm files that don't work correctly:

# List exports and imports
wasm-objdump -x module.wasm

# Disassemble
wasm-objdump -d module.wasm

# Print WAT form (verify Asyncify instrumentation)
wasm-opt --print module.so

# From JavaScript — inspect a side module
node -e "
  const fs = require('fs');
  const mod = new WebAssembly.Module(fs.readFileSync('module.so'));
  console.log('exports:', WebAssembly.Module.exports(mod).map(e => e.name));
  console.log('imports:', WebAssembly.Module.imports(mod).map(i => i.name));
"

Cross-reference symbol lists with:

• The ASYNCIFY_ONLY function list (main module)
• EXPORTED_FUNCTIONS in the Emscripten build flags
• SIDE_MODULE / MAIN_MODULE dynamic linking expectations

Diagnostic Cheat Sheet