Phyrexian unlife and life gain
You will not get any poison counters in that case. All damage is done simultaneously, and at the time you took damage you were at 1 life, not 0 or less life. So, you will take 5 normal damage, and be at -4 life. Normally you would lose now, but Phyrexian Unlife keeps you alive instead, and the next time you would take damage, that damage will be in the form of poison counters.

If you do gain life to above 0 again, then Phyrexian Unlife will again stop you from losing the next time you take damage.

Also, trample does not interact in anyway with this, so when you say "Excluding trample of course" it makes me think that there's a basic rule you aren't understanding about how combat or trample works.