@dobsun it is not an issue with phprad but your code. This
return isset($_num0[$nmber]) ? $_num0[$nmber] : $_mod10[$nmber] ."надесет";
is wrong, rather this is the right way to do such logic.
return isset($_num0[$nmber]) ? $_num0[$nmber] : "надесет";
You cannot be accessing an empty array after checking if it is empty.