Although nothing is certain, it is believed that one day Phobos will actually crash into Mars. This is because Phobos orbits Mars very quickly, much faster than Mars rotates, which brings it closer and close to the planet, however very, very slowly. It will continue to get closer and closer to Mars over the next millions of years, until it's close enough to Mars that it will break apart. It may end up forming into Mars, or it may become a ring around Mars.