Sum Up 3 previous values of an array in php

Easy like this ;)

$arr = array(2, 2, 3, 4, 5, 4, 1, 3, 2);
$sum = array();

echo 'Original:<br/>';

foreach($arr as $key => $a){
echo $a . '<br/>';

$s = ($arr[$key - 2] + $arr[$key - 1] +$arr[$key]);
$sum[] = $s;
}



echo '<br/>ValuesSum:<br/>';

foreach($sum as $key => $s){
echo $s . '<br/>';


The output of the code is:

Original:
2
2
3
4
5
4
1
3
2

ValuesSum:
2
4
7
9
12
13
10
8
6