change the value of the array

My Array :

PHP Code:

Array
(
    [0] => Array
        (
            [income_user] => 00002
            [income_country] => 1CN,1US,1US,1ID
        )

    [1] => Array
        (
            [income_user] => 00003
            [income_country] => 2US
        )

    [2] => Array
        (
            [income_user] => 00004
            [income_country] => 1US
        )
) 


I want to change the value of each income_country:

PHP Code:

foreach ($data as $key => $value)
{
    foreach (explode(',', $value["income_country"]) as $val)
    {
        preg_match('#([0-9]+)([A-Z]+)#',$val,$extrack);
        $a              =   $extrack[1];
        $b              =   $extrack[2];
        $c              =   isset($z[$b])?$z[$b]:0;
        $d              =   $c + $a;
        $z[$b]          =   $d;
        arsort($z);
    }
    $data[$key]["income_country"] = $z;
} 


Results :

PHP Code:

Array
(
    [0] => Array
        (
            [income_user] => 00002
            [income_country] => Array
                (
                    [US] => 2
                    [CN] => 1
                    [ID] => 1
                )
        )

    [1] => Array
        (
            [income_user] => 00003
            [income_country] => Array
                (
                    // should [US] => 2

                    [US] => 4 //?
                    [CN] => 1 //?
                    [ID] => 1 //?
                )
        )

    [2] => Array
        (
            [income_user] => 00004
            [income_country] => Array
                (
                    // should [US] => 1

                    [US] => 5 //?
                    [CN] => 1 //?
                    [ID] => 1 //?
                )
        )
) 


why is the result always added to each array?