Dr. or PhD

[Michael Sylvester]

 I was once told that a book written by an author with PhD after his/her
name(for example, Mark Zorro,PhD) would probably be more scientifically
credible
than a similar topic book written by one with Dr.before the name(for
example, Dr.Mark Zorro.
One assumption is that anyone can get called Dr. (Dr.Dre-the rap artist}
but a PhD assumes academic credentials.

What is your take on this?

Michael Sylvester,PhD
Daytona Beach,Florida