Based only on your PhD visa, you can not get Unemployment benefits. The period until 30th of june, 2018, this kind of visa does not give you right to work full-time in Denmark.
If you apply for establishment card and get one, the visa will be granted on the condition that you do not receive Unemployment benefits according to the graduate rule. I you begin to receive Unemployment benefits according to the graduate rule while at the same time having an establishment card, then the establishment card will be canceled.
So, it is not relevant for you to become a member of an A-kasse, before you
- have a visa (like establishment card) that gives you right to work full-time, and
- have a job or is self-employed.
You must have been member for at least 1 year, and have earned at least DKK 228.348 during the last 3 years (only membership periods count here), before you can have unemployment benefits.
For work-permit based on other schemes, it could be relevant to become member of an A-kasse after graduation. If you become member of an A-kasse at latest 2 weeks after graduation, then you will have the "graduate rights" for a period up to 2 years. So up to 2 years after graduation you can start to use your "graduate rights", and receive Unemployment benefits according to the graduate rule. However this requires you have a work-permit other than the Establishment Card.