Consider this example:

<?php
$zip = new ZipArchive;
$zip->open(...);

$zip->addFile('path/file', 'foo');
$zip->renameIndex(0, 'bar');

echo $zip->getNameIndex(0); // 'bar'

$zip->unchangeIndex(0);

echo $zip->getNameIndex(0); // 'false'
?>

Unless you call save() in between, the unchangeIndex() call reverts back to the initial state of the archive - where index '0' did not exist.

If you called save() after addFile() and then renamed the file, you would be able to revert/undo the name change.