I don't think it was in my course book!-II

According to this theory, mass depends on velocity v: , where is the "rest mass" - the mass at zero velocity. So we can write the energy in terms of the rest mass and the velocity: A photon has zero mass, but always travels at the speed of light, c.

For more details visit this

But while the photons are travelling with the speed of light they may gain energy and would have some mass because Sir Albert Einstein proved through his relativity theory that light coming from distant stars was deviated from its path by the sun's large gravitational force .

The solution to this is simple as a I read in a book that Photons are traditionally said to be massless.

Dude, this is so troll. A photon never has mass, light doesn't have mass. Plus, a photon never has rest mass because it travels at the speed of light. To be clear, just because you travel at the speed of light, doesn't mean you don't have mass, according to special relativity, you actually have undefined mass, or if you want to be that guy, infinite mass.