(PHP 5 >= 5.2.0, PECL zip >= 1.8.0)
ZipArchive::addEmptyDir — Add a new directory
$dirname
   )Adds an empty directory in the archive.
dirnameThe directory to add.
   Returns TRUE on success or FALSE on failure.
  
Example #1 Create a new directory in an archive
<?php
$zip = new ZipArchive;
if ($zip->open('test.zip') === TRUE) {
    if($zip->addEmptyDir('newDirectory')) {
        echo 'Created a new root directory';
    } else {
        echo 'Could not create the directory';
    }
    $zip->close();
} else {
    echo 'failed';
}
?>