Create Subscriptions, UserSubscriptions tables, you already have Users table (I assume).
insert all notification names into Subscriptions table you want to send, UserSubscriptions table will have UserId and the SubscriptionId.
Question
I want to develop a subscribe user module in PHP in which the user will get all the notifications about the activity of the subscribed user.
Solution
Create Subscriptions, UserSubscriptions tables, you already have Users table (I assume).
insert all notification names into Subscriptions table you want to send, UserSubscriptions table will have UserId and the SubscriptionId.