Could you explain -> function returns itself in php

Question

function func($x) {
İf ($x == 0) {
return 0;
} else {
return $x + func($x - 1);
}
}
echo func(6); //output : 21

Answer 1

It is recursion. It occurs when function can call itself. We can present this case such way: 6+5+4+3+2+1+0 = 21