In PHP how to perform addition of 15 days on today's date and storing the result in database?

In PHP how to perform addition of 15 days on today's date and storing the
result in database?

I'm working on PHP and I need that my code takes the current
date(sysdate), add 15 days to it and then store it to the database. How to
fetch system date, perform action on it and then store the result in
database?