Having a Ph.D. does not guarantee you for PR status in Singapore. Also, only S Pass and Employment Pass holders can apply no matter what profession you are working in. However, if you are an S Pass or E Pass holder with a Ph.D., it may have some positive impact on your PR application.
However ICA will look at your overall profile during evaluation. While there will be positive impact of your PhD degree, they will also look at immigration intent (i.e. the probability of you and your family really likely to settle in Singapore).