Is it possible to obtain a second PhD degree in Europe? As I have heard it is not possible to get a second PhD in Europe if you have obtained first PhD in Europe. Is it true? My 1st PhD is in Theoretical Physics which I won't find a fair job related to that. so I am trying to apply for second PhD in Mechanical engineering or applied Physics.
I am most familiar with the situation in Germany. There it is in fact possible to get more than one PhD degree. Different departments award different types of PhD degrees, depending on the academic field. In physics, you will typically earn a "Dr. rer. nat." (PhD in science), while in engineering you will get a "Dr.-Ing." (PhD in engineering). While most dissertation regulations will explicitly exclude the possibility that the same degree can be awarded twice, it is of course possible to earn a "Dr.-Ing." degree even if you already have a "Dr. rer. nat." or vice versa. Then you would have two PhD degrees.

In addition, there are honorary PhDs, which are awarded independently of and in addition to real PhD degrees. But I assume that your question wasn't aimed at those.

However, that it is possible to get two PhD degrees does not mean that it is a wise thing to do. To others, it may just be a sign that you didn't take the next steps in an academic career after getting your PhD, but didn't leave academia either.