Easy example for understanding:
<?php

$iterator = new ParentIterator(
    new RecursiveArrayIterator(
        array(array(1, 2, 3), 'A', 'B', 'C')
    )
);

foreach ($iterator as $recursive) {
    foreach ($recursive as $value) {
        echo $value . PHP_EOL;
    }
}
?>

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ParentIterator is just a RecursiveFilterIterator whos accept() method calls the RecursiveFilterIterator->hasChildren() method to filter itself.

Basically, it filters out leaf nodes. For example

This would yield all files and directories
<?php
$rdi = new RecursiveDirectoryIterator(__DIR__);
$iter = new RecursiveIteratorIterator($rdi, RecursiveIteratorIterator::CHILD_FIRST);
?>

wrapping that in a ParentIterator would filter it down to just directories
<?php
$rdi = new RecursiveDirectoryIterator(__DIR__);
$iter = new RecursiveIteratorIterator($rdi, RecursiveIteratorIterator::CHILD_FIRST);
$dirsOnly = new ParentIterator($iter);
?>