docs: improver worker docs, and add internals docs

Author: dunglas

docs/worker.md

@@ -168,3 +168,38 @@ $handler = static function () use ($workerServer) {
 
 // ...
 ```
+
+Most superglobals (`$_GET`, `$_POST`, `$_COOKIE`, `$_FILES`, `$_SERVER`, `$_REQUEST`) are automatically reset between requests.
+However, **`$_ENV` is not reset between requests** for performance reasons.
+This means that any modifications made to `$_ENV` during a request will persist and be visible to subsequent requests handled by the same worker thread.
+Avoid storing request-specific or sensitive data in `$_ENV`.
+
+## State Persistence
+
+Because worker mode keeps the PHP process alive between requests, the following state persists across requests:
+
+- **Static variables**: Variables declared with `static` inside functions or methods retain their values between requests.
+- **Class static properties**: Static properties on classes persist between requests.
+- **Global variables**: Variables in the global scope of the worker script persist between requests.
+- **In-memory caches**: Any data stored in memory (arrays, objects) outside the request handler persists.
+
+This is by design and is what makes worker mode fast. However, it requires attention to avoid unintended side effects:
+
+```php
+<?php
+function getCounter(): int {
+    static $count = 0;
+    return ++$count; // Increments across requests!
+}
+
+$handler = static function () {
+    echo getCounter(); // 1, 2, 3, ... for each request on this thread
+};
+
+for ($nbRequests = 0; ; ++$nbRequests) {
+    if (!\frankenphp_handle_request($handler)) break;
+}
+```
+
+When writing worker scripts, make sure to reset any request-specific state at the beginning of each request handler.
+Frameworks like [Symfony](symfony.md) and [Laravel Octane](laravel.md) handle this automatically.