Help with Function Return example

Question

I try to understan by myself but I just could not do it. I need help with the process of this example

<?PHP

function
math($t)
{
if($t==0)
return 0;
return $t + math($t - 1);
}
echo math(6);

?>

The result is 21

Why?

Answer 1

In this example is used recursion => a function which calls itself. It's defined the base case ($t==0 ). The function will call itself until the base case is reached. So the result is 6+5+4+3+2+1 => 21.