feat(php): compile extensions from source when unavailable in static builds

[eddmann]

Summary

When a requested extension is not available in the pre-built static PHP binaries (common or bulk tiers), phpx should fall back to compiling the extension from source instead of failing.

Current Behavior

Extensions are specified in the // phpx metadata block:

// phpx
// php = ">=8.2"
// extensions = ["redis", "imagick"]

phpx checks if extensions are available in the "common" or "bulk" static PHP builds from static-php.dev. If an extension isn't available in either tier, resolution fails with:

extension 'foo' not available in static PHP builds

See internal/index/index.go:433 in RequiredTier().

Proposed Behavior

Instead of failing, phpx should:

1. Select the best available static PHP build (common/bulk) for extensions that are available
2. For extensions not in static builds, download source from PECL and compile them
3. Cache compiled extensions in ~/.phpx/extensions/<php-version>/<ext-name>/
4. Load compiled extensions via -d extension=/path/to/ext.so

Implementation Considerations

• Source retrieval: Download from PECL (https://pecl.php.net/get/<ext>-<version>.tgz)
• Compilation: Run phpize, ./configure, make against the static PHP headers
• Caching: Store compiled .so files keyed by PHP version and extension version
• Dependencies: Some extensions need system libraries (e.g., ImageMagick for imagick)
• Build tools: Requires gcc, make, autoconf - should detect and report if missing
• Version selection: Default to latest stable PECL version, allow pinning via extensions = ["redis:5.3.7"]

Example

// phpx
// php = ">=8.3"
// extensions = ["redis", "some-obscure-ext"]

• redis → available in bulk tier, use static build
• some-obscure-ext → not in static builds, compile from PECL

Additional Context

• This extends the existing extension system without changing the user-facing syntax
• Compiled extensions would be cached alongside PHP binaries in ~/.phpx/
• Should gracefully handle compilation failures with clear error messages