Going through chinese customs

Hi !! I will be travelling in Beijing next week and I don't understand the process at the customs.Do I have to declare my iphone or sth that i may buy during the layover in Rome(perfumes,souvenirs,ecc)? And when leaving China,do i have to declare what I have bought (laptop,mp3,phone,clothes,bags,etj real/fake).Both entering and leaving I'm thinking of going through the red line just to be sure.
Thank you in advance!!