Convert Java HashSet to PHP

Question

1  for (int i=0; i<paymentList.size(); i++) {
2     listClassDate.addAll(
3         getClassDateList(paymentList.get(i).getClassIdx())
3     );
5  }
6
7  HashSet tmpClassDate = new HashSet(listClassDate);
8  listClassDate = new ArrayList<>(tmpClassDate);
9
10  Collections.sort(listClassDate);
11
12  for (String classDate : listClassDate) {                            
13     contentsList.addAll(getSeriesClassBook(
14         selectedSeries.getSeriesCode(), 
15         classDate, 
16         paymentList
17       )
18    );
19  }

I'm converting a project from Java to PHP, but since I know little about Java, quite hard to convert it. My problem on the code above are the following:

1. In line 7, what the HashSet does to listClassDate? How can i make this in PHP?
2. In line 12, for (String classDate ... . I haven't tried "strings in PHP for loop" because i'm assuming classDate is always array in PHP.
3. The classDate is only my problem in the parameter, because that is a Java thing.

Parameters for getSeriesClassBook() method:
getSeriesClassBook(String seriesCode, String classDate, List paymentList)

Answer 1

1) It's a quick way to remove duplicates from the list. They makes a set by copying the list, which automatically removes duplicates. They then put it back into a list again.

2) It just a foreach loop. The same concept exists in PHP .

I don't understand question 3.