Variables initialized as strings and then used as arrays cause fatal errors if PHP >= 7.1

I can confirm the drag n drop of events in calendar view is an existing bug for vtiger 7.1 using php 7.1. So I think that makes vtiger 7.1 not compatible with php 7.1

The problem is in modules/Calendaractions/DragDropAjax.php:

The error is:

That error was corrected in #1058 (closed) but there are many more initializations like that in the same files.