Как найти разницу между соседними элементами массива?

<?php
$a = array(
    array(
        "year" => 0,
        "serial" => 0
    ),
    array(
        "year" => 0,
        "serial" => 13000
    ),
    array(
        "year" => 0,
        "serial" => 13640
    ),
    array(
        "year" => 0,
        "serial" => 14280
    )
);

$prev = $a[0]['serial'];

for($i = 1; $i < count($a); $i++) {

    $diff = $a[$i]['serial'] - $prev;
    $prev = $a[$i]['serial'];
    $a[$i]['serial'] = $diff;
}
print_r($a);

Array
(
    [0] => Array
        (
            [year] => 0
            [serial] => 0
        )

    [1] => Array
        (
            [year] => 0
            [serial] => 13000
        )

    [2] => Array
        (
            [year] => 0
            [serial] => 640
        )

    [3] => Array
        (
            [year] => 0
            [serial] => 640
        )

)