On Difficult terrain each square counts as 10 ft movement, this prevent 5 foot steps so player will have just a single attack unless they didnt move at all.
The question will be if squares count as 10ft of movement then moving a single square would provoke an attack of opportunity? I mean because it would count as moving from a threat range.
Yes it would provoke an attack of opportunity.
Assuming no ability to ignore difficult terrain there is no 5' step in difficult terrain. So, no full attack with ay movement, and leaving a threatened square provokes.*
*Unless using the withdraw action.
Yup. You would need to spend a move action to move (since it's more than 5 feet), and if you're leaving a threatened square then it provokes.