Physical path of directories

I need to know the physical path of the directories on my webserver for some php code I am using, and I wasn’t exactly sure what they are. Where would I find that info?

For example (the audio directory, which is at “/home/username/”

Also, how do I know if that directory is writable by the webserver?

That’s it. And if the directory is writable by you, then it’s writable by the web server since it’s running as you.