The addition is done on this line
$sum = $sum + $number;
Look at the sequence above.
$sum starts as 0.
First 1 is added to $sum (1+0), so $sum is now 1.
Next 2 is added (1+2), $sum is now 3.
Then 3 is added (3+3), $sum is now 6.
4 does not get added.
The result is 6.