What is the meaning of "_="?

[] means push - put the given argument as a new element on the end of the array. That means that $ACTIVITYGROUPS is an array*.

$arr = array();
$arr[] = 1;       // Put 1 in position 0
$arr[] = "a";     // Put "a" in position 1
$arr[] = array()  // Put a new, empty array in position 2

As stated by the PHP docs, array_push has the same effect as [].

---

* If it's not an array, using [] will give you a syntax error:

Warning: Cannot use a scalar value as an array in test.php on line 4

In many languages the [] notation stands for an array. Is the same as php's array_push(): it pushes an element in the variable that has [] at the end.

If the variable is null, you can consider the square brackets like a declaration of an array.

The same notation of push applies to Javascript, for example. When using it like $var[] = 'a'; what happens is the same as array_push() I was talking above. Just finds the next position in the array and adds there your value.

An array can be walked with for, foreach, while, do while and you can check it's contents with print_r() or var_dump() functions.