I will create a mediaplayer by using C++ and Qt 4. Here it says:
The Qt backend on Linux uses GStreamer (minimum version is 0.10), which must be installed on the system. At a minimum, you need the GStreamer library and base plugins.
But I can't use Phonon even I added it to .pro file. When I include #include <phonon> it can't find Phonon. How will I be able to use Phonon? Do I need to do any installation or something else?
When you ran configure to install Qt, did you somehow disable Phonon support?
-phonon and -phonon-backend are enabled by default.
Qt documentation notes:
Phonon is built if a decent C++ compiler is used.
Are you using an up-to-date compiler?
You might try clearing your current configuration and running configure again:
make confclean
configure