Need to update MySQL and SAP database at the same time from a Form

Yes, you can do that but if you are thinking of making the changes in the existing PHP script so that it takes care of both the tasks. Then it will be quite tough.

But if you want to have two operations in parallel then you can have it.

You can go for writing a web service which can do the DB update activity. You can also try using the SAP API to do the updates. SAP API has the most common and used methods to do the updates.

So you might be able to find the ones for your requirement.

You can also try to use SAP Connection RFC. It is widely used. You can find more details here: 

Github php-sapnwrfc

At last, I will suggest going for either RFC or API.