为什么这个递归函数会像这样工作？

Question

I am trying to learn recursion and have written a function that takes a number and count it down to zero and then count it up to the original number again, the function works, but what I don't understand is why it works as it does. I understnad why the first print after the else statement prints: "5 4 3 2 1" and then the number is 0 and the if statement prints: "0". It is after this scenario I don't understand since now the function enters the second print after the else statement and prints: "1 2 3 4 5" which is very strange to me. I would really appreciate if someone could explain this to me.

<?php
function rec_downandup($num){
    if($num == 0){
        print '0 ';
    }else{
        print $num.' ';
        rec_downandup($num-1);
        print $num.' ';
    }
}
rec_downandup(5);
?>

Output

5 4 3 2 1 0 1 2 3 4 5

Answer 1

When you take a closer look, it should become clear.

print $num.' ';
rec_downandup($num-1);
print $num.' ';

For your first input, you will get

print 5.' ';
rec_downandup(4);
print 5.' ';

and after that call it is

print 5.' ';
print 4.' ';
rec_downandup(3);
print 4.' ';
print 5.' ';

So it appears the functions counts down and up, but actually it just counts down and places each number twice - the second time in reverse order so it appears to be counting up.