La clase ParentIterator

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);
?>