Php 在foreach中有条件地向数组添加元素

我目前有以下代码用于填充要返回的注释列表：

foreach ($comments as $comment)
{
    $f_comments[] = array (
        'comment_id'    => $comment['comment_id'],
        'comment_body'  => $comment['comment_body'],
        'user_id'       => $comment['user_id'],
        'user_avatar'   => $comment['user_avatar'],
        'user_username' => $comment['user_username'],
        'timeago'       => formatter_common_format_time_string($comment['added']),
    );

    if (!empty($comment['user_picture']))
    {
        $f_comments[]['user_picture'] = $comment['user_picture'];
    }
}

问题是，返回的是：

"comments":[{"comment_id":9386,"comment_body":"Comment","user_id":46542,"user_avatar":"9","user_username":"TestUser","timeago":"about 2 hours ago"},{"user_picture":"ec237f517bc26b27d8e790c3a5d125841321552075"}]

…而我希望用户_图片包含其余结果，如下所示：

[{"comment_id":9386,"comment_body":"Comment","user_id":46542,"user_avatar":"9","user_username":"TestUser","timeago":"about 2 hours ago", "user_picture":"ec237f517bc26b27d8e790c3a5d125841321552075"}]

---

…但是我看不出有什么办法。我对PHP很陌生，所以我可能遗漏了一些明显的东西。有人知道怎么回事吗？

在评论中添加索引：

foreach ($comments as $i => $comment)
{
    $f_comments[$i] = array (
        'comment_id'    => $comment['comment_id'],
        'comment_body'  => $comment['comment_body'],
        'user_id'       => $comment['user_id'],
        'user_avatar'   => $comment['user_avatar'],
        'user_username' => $comment['user_username'],
        'timeago'       => formatter_common_format_time_string($comment['added']),
    );

    if (!empty($comment['user_picture']))
    {
        $f_comments[$i]['user_picture'] = $comment['user_picture'];
    }
}

---

或者只创建一个变量$i=0在foreach之前增加它，然后再关闭它$i++

在评论中添加索引：

foreach ($comments as $i => $comment)
{
    $f_comments[$i] = array (
        'comment_id'    => $comment['comment_id'],
        'comment_body'  => $comment['comment_body'],
        'user_id'       => $comment['user_id'],
        'user_avatar'   => $comment['user_avatar'],
        'user_username' => $comment['user_username'],
        'timeago'       => formatter_common_format_time_string($comment['added']),
    );

    if (!empty($comment['user_picture']))
    {
        $f_comments[$i]['user_picture'] = $comment['user_picture'];
    }
}

或者只创建一个变量$i=0在foreach之前增加它，然后再关闭它$i++