Your web server and thereby your PHP scripts are running as a certain user, typically www-data
or something similar. The folder needs to be writable and readable by that user, or by a group that user is part of. Other users/groups do not require access to that folder, so you shouldn't give them any.
Assuming the folder is owned by www-data
(or other equivalent user your web server/PHP scripts are running as), the minimum necessary permission is 700
(full access to owner, no permissions for group or others). Depending on your specific setup you may or may not need to give further access to others, only you know. 744
is still pretty "safe", since it only gives read access to other users and I'm assuming the data is not exactly secret to begin with.