Where to put declaration in phd thesis?
Not every university demands such a declaration (e.g., stating that it is your own work, and all external sources have been properly cited, referenced, and acknowledged). I had one in my master thesis, but not in my PhD thesis (performed at a different university). Just follow the university rules. If they demand a declaration, then typically there is a verbatim text that you have to put in.

If your university does not prescribe where it should come, but does demand that you have a declaration, then you are free to put it anywhere. It is, however, wise to follow the convention of your university, and put it where others have put it. As @scaaahu indicated, you can go to the library and look what others did.

(Btw, I have seen it typically somewhere at the beginning.)