Interactive code examples fail to run with `"parallel\Runtime" not found`

From manual page: https://php.net/class.parallel-future

---
https://www.php.net/manual/de/class.parallel-future.php
Running example code from `Run code` results: 

```
Fatal error: Uncaught Error: Class "parallel\Runtime" not found in php-wasm run script:2
Stack trace:
#0 {main}
  thrown in php-wasm run script on line 2
```