Cannot find a way to get the document name

[rgpublic]

I cannot for the life of me find a way to extract the document name:

<?php
use lsolesen\pel\PelJpeg;
use lsolesen\pel\PelTag;

$jpeg=new PelJpeg('samplepic-doctitle.jpg');
$exif=$jpeg->getExif();
$tiff=$exif->getTiff();
$ifd=$tiff->getIfd();
print_r($ifd->getEntry( PelTag::DOCUMENT_NAME));

I see the name, though, if I check with GNOME's file manager or other apps: