I am plaining to use Ubuntu for developing Android and php application which I will sell in market. As far as I understand Ubuntu falls under GNU GPL license.
In this case do I need to make my source code open for all?
Work done on a particular operating system is not derived from it and therefore is not covered by it's license (or more properly license of any of it's components, since operating system consists of many components with different licenses).
An application is derived from libraries it links to, but Android application only links libraries that come from the Android SDK and possibly NDK and neither of those limits what you can do with the application. You would have to be careful about license of any 3rd party library you'd want to include though.
The answer will depend on exactly what you're selling. Are you reselling copies of Ubuntu, or selling modified Ubuntu software? Then the Ubuntu software license becomes important.
When developers simply use Ubuntu to make something else, we're using it as a tool just like a pencil or a calculator. And Ubuntu's GPL license doesn't apply to an independent, stand-alone product we've made with it.
So, are you selling an Android app in an online app store? Then check out the terms in the developer contract. And the PHP app — are you selling files with PHP in them to someone? Or on the other hand, will the PHP code never leave your server because it's a web app? These are two different scenarios. Either way, when a developer completely writes source code from scratch, they own the copyright to it and can do what they like with it. If, though, it requires, embeds or links to other libraries when it runs, then there might be obligations enforced by the GPL or another open-source license.