php遍历时修改 传地址

正确：

<?php
    $arr = array(
        "a" => array(
            "a" => "1",
        ),
    );
    foreach ($arr as $a => &$trdata) {
        $trdata['b'] = $trdata['a'];
    }
    var_dump($arr);
?>

错误：

<?php
    $arr = array(
        "a" => array(
            "a" => "1",
        ),
    );
    foreach ($arr as $a => $trdata) {
        $trdata['b'] = $trdata['a'];
    }
    var_dump($arr);
?>