https://github.com/sorvani/freepbx-helper-scripts/blob/master/Extensions_to_Yealink_AddressBook/ylab.php is still working as of 11.3.2022. My FreePBX extension names contained the "&" which is a reserved symbol in XML and will throw an error in the script. If you run into this, the fix is below.
Simple fix is to edit this line:
echo " <Name>" . $extension['description'] . "</Name>\n";
to this:
echo " <Name>" . htmlspecialchars($extension['description']) . "</Name>\n";