Restrict access to all files outside public_html

We have several accounts that need to upgrade their websites and as a result access needs to be given to web developers. As a security precaution, we would like to restrict access for the said web developers to public_html folder only. (Primary concern is that they shall not have access to Mail folders) Although we can restrict access through ftp to public_html, any PHP script uploaded to public_html has access to all files within the account, including mail. So Mail can be read and even deleted through PHP script execution. Could someone advice if such a restriction is possible and how can it be achieved ? Thank you in advance.