SplPriorityQueue::compare

At this time, the documentation sais "Note: Multiple elements with the same priority will get dequeued in no particular order."If you need elements of equal priority to maintain insertion order, you can use something like:<?phpclass StablePriorityQueue extends SplPriorityQueue {    protected $serial = PHP_INT_MAX;    public function insert($value, $priority) {        parent::insert($value, array($priority, $this->serial--));    }}?>

To create a min-heap priority queue, and have extract() give you the lowest priority:class MinPriorityQueue extends SplPriorityQueue {    public function compare($priority1, $priority2) {        if ($priority1 === $priority2) return 0;        return $priority1 > $priority2 ? 1 : -1;     }}