I mean a link inside "in app browser" page - may a PHP page from my PHP site,... can link to a link of a page in app
// whereas index.html be the first page of the Cordova App...????
You cannot link to a page within your app from outside the app. There is no web server in your app, all of the pages that are within your app are file based and are not accessible outside the webview in which they run. When you use inAppBrowser it only has access to pages outside of the webview, that is its purpose.