How to increase the value of a unix timestamp stored in a variable by X hours?

How to increase the value of a unix timestamp stored in a variable by X
hours?

I have a php function that creates a unix timestamp and stores it in a
variable. Let's call that variable $timestamp. I'm looking for a function
that will allow return two new variables that are a each a different
number of hours later than the original timestamp. Is there a way to do
this?