The SplMinHeap class

I experimented what happens when arrays are inserted:

<?php
$heap = new SplMinHeap();
$heap->insert([22,333]);
$heap->insert([2,33]);
$heap->insert([222,3]);

var_export($heap->extract());
echo '<br>';
var_export($heap->extract());
echo '<br>';
var_export($heap->extract());
?>

Output:

array ( 0 => 2, 1 => 33, )
array ( 0 => 22, 1 => 333, )
array ( 0 => 222, 1 => 3, )

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