Good question. It's interesting because that's the actual destination folder in which your PHP files reside, but not the correct destination of the web server address in which they are hosted. So I'm assuming this would only work for HTML files. It'd be like trying to view a PHP file that's located on your desktop through a browser, it wouldn't work. But if it were an HTML file, it would work because it doesn't require a server to execute the PHP code.
You think Espresso even has a built-in feature to fix this? Because isn't it primarily an interactive CSS and HTML editor? Anyway, have you looked through all the preferences in Espresso --> Preferences or whatever the name of the settings menu item is? Perhaps there is some setting that allows you to change the path for PHP files to be the corresponding web server root address..