Note the definition of SplMaxHeap::compare() is different from SplMinHeap::compare()
(PHP 5 >= 5.3.0, PHP 7)
SplMinHeap::compare — Compare elements in order to place them correctly in the heap while sifting up.
The value of the first node being compared.
The value of the second node being compared.
Result of the comparison, positive integer if
value1 is lower than
value2, 0 if they are equal, negative integer otherwise.
Having multiple elements with the same value in a Heap is not recommended. They will end up in an arbitrary relative position.