This is dumb but can someone explain why this outputs 10. Im confused

Question

https://code.sololearn.com/wlY8c208NDr6/?ref=app

Answer 1

left is a and right is i:

0+0=0;
0+1=1;
1+2=3;
3+3=6;
6+4=10;

Answer 2

$a is summation of numbers 0 to 4
it's like this code
$a = 0
$a = $a + 0
$a = $a + 1
$a = $a + 2
$a = $a + 3
$a = $a + 4
echo $a