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:

<?php

class 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; 
    }
}