Warning SMS before actual date to Client using PHP

Let's assume. I have a customer who purchases some goods and gives me a half payment and tells me that I will pay the remaining payment on this specific date.

So for this, I want to make a program which sends message automatically to the client before the actual date. This message act like a warning that 2 days are remaining or 3 days. I want to do this in PHP.

How is this possible I mean what step I have to do after using SMS Gateway?