Sixteen year old is disconnecting from reality

My 16-year-old daughter claims that she still has imaginary friends. I did not believe her at first, but then I witnessed her talking to herself one day. I mean a true conversation. I asked her about these friends of hers. She told me that she had more than 5.

There is one thing I did not tell you yet, she said that for every group of friends she has another name, age, basically life. She has also created these imaginary worlds that she just suddenly falls into. I asked her to stop talking to these friends, but less than a week later she told me she could not live without them.

Now, my daughter has begun to even use the names that she has created. I mean when she introduces herself, sometimes she says the wrong name. She is slowly but surely backing away from the real world, and that is scaring me. I mean she does not even have any real friends anymore. I need to know if my daughter has a mental disorder. If yes I need to what the mental disorder is and how to correct the problem. If not I need to know is this a phase or what?