Can some one help me to understand this PHP question please? $result = 0; and $i = 0; how can their addition be 10???

Question

function func($arg) {
$result = 0;
for($i=0; $i<$arg; $i++) {
$result = $result + $i;
}
return $result;
}
echo func(5);

Answer 1

1+2+3+4=10

The for loop add the numbers

http://www.w3schools.com/php/php_looping_for.asp