laravel - php, why use objects? Can't I use an array instead?

<code><?php
//奥运奖牌榜,依次按金牌,银牌,铜牌的数目进行降序排序.
header('Content-Type: text/plain; charset=utf-8');
$arr = array(
    '中国' => array(
        '金牌' => 8,
        '银牌' => 3,
        '铜牌' => 6,
    ),
    '俄罗斯' => array(
        '金牌' => 3,
        '银牌' => 6,
        '铜牌' => 3,
    ),
    '美国' => array(
        '金牌' => 6,
        '银牌' => 8,
        '铜牌' => 8,
    ),
    '澳大利亚' => array(
        '金牌' => 4,
        '银牌' => 0,
        '铜牌' => 4,
    ),
    '意大利' => array(
        '金牌' => 3,
        '银牌' => 4,
        '铜牌' => 2,
    ),

);

// 实现 ORDER BY
foreach($arr as $k => $v) {
    $sort['金牌'][$k] = $v['金牌'];
    $sort['银牌'][$k] = $v['银牌'];
    $sort['铜牌'][$k] = $v['铜牌'];
}
array_multisort(
    $sort['金牌'], SORT_DESC, 
    $sort['银牌'], SORT_DESC, 
    $sort['铜牌'], SORT_DESC, 
    $arr);
var_export($arr);

// 实现 GROUP BY (看看'金牌'字段都有哪些值)
$tmp = array();
foreach($arr as $k => $v) {
    if(in_array($v['金牌'], $tmp) === FALSE) {
        $tmp[] = $v['金牌'];
    }
}
var_export($tmp);</code>