First off, the requirements and difficulty of a Ph.D. qualifying exam varies greatly between universities, within departments in a given university, and even by examining committee within a department!
I consider myself having gotten off easily, as my candidacy examination consisted of writing a research report of about 60 pages in length on my research topic and an oral exam of about 90 minutes by my examining committee (my two co-advisors, the examiner internal to the department, and the examiner outside the department). The contents of the report was a literature survey, methodology, some numerical/experimental results, and a discussion on the originality of the research/proposed future work. This report was approximately the first three chapters of my Ph.D. thesis. In this case there was no examination of material outside of my research project, as the intent of the candidacy examination in my case was to determine the likelihood of succeeding in the research component of a Ph.D. program. I had a relatively friendly examining committee.
Notwithstanding the relatively light requirements of my Ph.D. candidacy examination, the writing of the report alone (i.e., not including the research) took me more time than the preparation time for my PE and FE exams combined. I think (that at least in my case) preparing for the PE exam required less effort and was less uncertain than preparing for my Ph.D. candidacy examination because the topics on the PE exam were well defined and required no original research. By way of comparison, the PE exam was straightforward relative to the Ph.D. candidacy examination. As has been stated above, the intents of the two examinations are very different.
I've found that in the market I'm practising in, holding a Ph.D. in Engineering while working in a consulting environment adds weight and credibility to the opinions that I express. I've also found that it has added an estimated 40% premium on the renunciation received compared to holding a M.Sc./M.Eng. and even more compared to a B.Sc. A word of caution is that consulting is different from a research environment in that you need to deliver something usable to the client in a cost-effective and reasonable time frame, i.e., you can't turn jobs into "research projects."
Be aware also that the reasons for pursuing a PE license are different for the reasons for pursuing a Ph.D. The former is a legislative requirement to practice engineering, while the latter is an advancement of a body of knowledge.
Hope that helps.