You will be able to mention your PhD studies in question 8 of IMM5669E, personal history.
and this is from the instruction guide IMM7000:
Question 8
Provide the details of your personal history since the age of 18, or the past 10 years, whichever is most recent.
Start with the most recent information. Under “Activity”, write your occupation or job title if you were working. If you were not working, provide information on what you were doing (for example: unemployed, studying, travelling, retired, in detention, etc.). If you were outside your country of nationality, indicate your status in that country.
Note: Please ensure that you do not leave any gaps in time.
Also for this question 8: if we have had a trip like 1 week or so, shall we mention it here or it is enough to be mentioned in "Your Travels" form and no need to cut the time just for vacations here?