We have talked about using Views Custom Field which really makes Views more versatile. In the custom PHP field, you can get the data of other fields from $data. But the field names under the $data may changed later as you modify the view.

A workaround is to get the field data from $data using the field_id.

$data->{$this->view->field['field_id']->field_alias}

You can get the field_id by mouseover the field name in fields region of Views UI.

Done =)

Reference: field name in $data gets changed after adding new CCK field