As far as I read about it today, you can't debug anything else than node.js, JavaScript and TypeScript at the moment, but they said they want to add new languages which you can debug. The editor is still in development. Nevertheless, I don't think there will be a php debugger in the future since php is serverside, so you can't debug it on your client alone.
If you want to debug php, I can recommend xDebug.
Now, it is possible to debug with VS code. You need to install XDebug and php-debug extension for VScode.