There are two types of inverse numbers in this context: additive
and multiplicative.
An additive inverse number is a number that's had its sign
flipped: positive becomes negative, and negative becomes positive.
The PHP code for this would be:
$result=-$number;
A multiplicative inverse number is a number that's been divided
into 1. So 5 becomes 1/5, and 1/5 becomes 1/1/5, or 5. The PHP code
for this would be:
$result=1/$number;