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, )