How to read code with recursion?

Question

PHP

I am learning php. Got code:

spoiler

function factor($n)
{
if ($n <= 0) return 1;
else return $n * factor($n — 1);
}
echo factor(20);

It's not clear to me why subtract 1:
$n * factor($n — 1);

Answer 1

Because the factorial is the product of all positive numbers up to the current one. Starting from the desired number, we multiply it by the previous one at each iteration until we get 0, which will return 1.
For 5, these are the steps:
5 4 3 2 1 0