Let's assume you have a variable with a negative value, and you'd like to make it a positive value. Here's a simple way:
Basically, this approach negates the variable, so you can do the opposite as well:
Note that this will not change $a unless you reassign the result to the variable.
If you just want to make sure a variable has a positive number, try Abs():
To make sure your variable is always negative, you can combine both: