To expand on gom's comment, SplMinHeap will also take into account the subsequent elements of the array you inserted, if the elements before that are equal.
<?php
$heap = new SplMinHeap();
$heap->insert([0, 10]);
$heap->insert([0, 30]);
$heap->insert([0, 15]);
while (!$heap->isEmpty()) {
[$a, $b] = $heap->extract();
echo "a:$a, b:$b\n";
}
echo "---\n";
$heap->insert([0, 10, 100]);
$heap->insert([0, 10, 300]);
$heap->insert([0, 10, 150]);
while (!$heap->isEmpty()) {
[$a, $b, $c] = $heap->extract();
echo "a:$a, b:$b, c:$c\n";
}
?>
will output:
a:0, b:10
a:0, b:15
a:0, b:30
---
a:0, b:10, c:100
a:0, b:10, c:150
a:0, b:10, c:300
