How worth is it doing a PhD straight after taking the first degree? Does having a PhD help in getting a job afterwards? Will the starting salary automatically be higher than the one offered to a graduate?

You need to ask yourself what kind of work you want to be doing. If you wish to be a researcher and/or work in academia then a PhD is pretty much essential. If not, it might not be necessary for you to gain a PhD degree.

The job would obviously be pertinent to the area of research of the PhD (engineering). Would a PhD boost my initial salary in an engineering job?

Not my field, I'm afraid. You could probably find out easily enough by searching on the web for the kind of job you'd like to do in the future, and looking at the required qualifications and salaries involved. Have you asked your first degree tutors or University careers service for advice on this matter?

You should really speak to your course tutor about this. The careers service will also be helpful. The chances are that with a PhD you will earn more than a new graduate, but probably not more than a graduate who's already been working for three years (while you were doing your PhD). However it is the kind of work you want to get into which is affected by a PhD. The door will be closed to many research jobs (particularly in academia) without PhD level qualifications.

DO NOT go for a PhD if you don't wish to teach and/or do research in a university. But if you still go for it, complete it and do not expect to get anything in return, just because you have a PhD. Also remember that, after a 4-5 year industry experience, if you want to teach, it is extremely difficult or at times impossible to get into academia. This is all from my experience.

A Master's degree is good enough for industry.

PVM, PhD