SplHeap::compare
(PHP 5 >= 5.3.0, PHP 7)
SplHeap::compare — Compare elements in order to place them correctly in the heap while sifting up
Description
abstract protected SplHeap::compare ( mixed $value1 , mixed $value2 ) : int
 Compare value1 with value2. 
Throwing exceptions in SplHeap::compare() can corrupt the Heap and place it in a blocked state. You can unblock it by calling SplHeap::recoverFromCorruption(). However, some elements might not be placed correctly and it may hence break the heap-property.
Parameters
-  value1
-  The value of the first node being compared. 
-  value2
-  The value of the second node being compared. 
Return Values
 Result of the comparison, positive integer if value1 is greater than value2, 0 if they are equal, negative integer otherwise. 
Note:
Having multiple elements with the same value in a Heap is not recommended. They will end up in an arbitrary relative position.
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    https://www.php.net/manual/en/splheap.compare.php