docs: add php subsystems section

docs/src/SUMMARY.md

@@ -10,6 +10,7 @@
 - [Python](./subsystems/python.md)
 - [Node.js](./subsystems/node.md)
 - [Haskell](./subsystems/haskell.md)
+- [PHP](./subsystems/php.md)
 
 # Concepts
 - [Architectural Considerations](./intro/architectural-considerations.md)

docs/src/subsystems/php.md

@@ -0,0 +1,23 @@
+# PHP subsystem
+
+> !!! PHP support is a work in progress, and it is not yet usable (a
+> builder is missing). You can track the progress in
+> [nix-community/dream2nix#240](https://github.com/nix-community/dream2nix/issues/240).
+
+This section documents the PHP subsystem.
+
+## Translators
+
+### composer-lock (pure)
+
+Translates `composer.lock` into a dream2nix lockfile.
+
+### composer-json (impure)
+
+Resolves dependencies in `composer.json` using `composer` to generate a
+`composer.lock` lockfile, then invokes the `composer-lock` translator to
+generate a dream2nix lockfile.
+
+## Builders
+
+None so far.