How to set a dynamic range by fixed number

I am trying to set a range by a fixed number for an example.

$fixed = 5;
$value = 7;

So if my value is 7 it will be in a range from 5 to 10

I was trying the old fashion method to add + 5 to the value, but it goes out of the range. (7 + 5 = 12) and it has to be in range between 5 to 10.

$value = 19; // range in 15 to 20
$value = 23; // range in 20 to 25

Any idea?