这个问题能给出正确的解答吗，问题区好几个人问，都没给出解决的办法

<?php

namespace app\index\controller;

use app\index\model\Tag as TagModel;

use app\index\model\Topic as TopicModel;

use app\index\model\TopicTag as TopicTagModel;

use think\Controller;

class Topic extends Controller {

    public function newTopic() {

        if (request()->isPost()) {

            $postData = input('post.');

            $topic = new TopicModel();

            $user = session('user');

            $topic->title = $postData['title'];

            $topic->category_id = $postData['category_id'];

            $topic->content = $postData['content'];

            $topic->user_id = $user->id;

            $topic->created_at = intval(microtime(true));

            $topic->save();

            //处理标签

            $tags = $postData['tags'];

            foreach ($tags as $tag) {

                if (is_numeric($tag)) {

                    $this->createTopicTag($tag, $topic->id);

                    continue;

                }

                $newTag = $this->createTag($tag);

                $this->createTopicTag($newTag->id, $topic->id);

            }

            $this->success('恭喜！帖子创建成功！');

        }

        $this->assign([

            'user' => session('user'),

            'category' => config('category'),

            'tags' => TagModel::all()

        ]);

        echo $this->fetch('new_topic');

    }

    private function createTopicTag($tagId, $topicId) {

        $topicTag = new TopicTagModel();

        $topicTag->tag_id = $tagId;

        $topicTag->topic_id = $topicId;

        $topicTag->save();

    }

    private function createTag($name) {

        $tag = new TagModel();

        $tag->name = $name;

        $tag->save();

        return $tag;

    }

    public function detail() {

        $topicId = input('get.id');

        $topic = TopicModel::getTopic($topicId);

        $user = session('user');

        $this->assign([

            'user' => $user,

            'topic' => $topic,

            'topicTags' => TopicTagModel::getTopicTagsByTopicId($topic->id),

            'categoryNames' => getCategoryNames($topic->category_id),

        ]);

        echo $this->fetch('detail');

    }

}

common.php文件

<?php

// 应用公共文件

function getCategoryNames($id) {

    $category =[

        'imooc' => [

            1 => '站务与公告',

            2 => '反馈',

            3 => '使用指南',

        ],

        'Mobile' => [

            8 => 'Android',

        ],

        'Lifestyle' => [

            10 => '意欲蔓延',

        ],

        'Technology' => [

            5 => '程序猿',

            6 => '分享与创造',

        ],

    ];

    //$category = config('category');

    foreach($category as $key => $cate) {

        foreach($cate as $category_id => $categoryName) {

            if ($category_id == $id) {

                return [$key, $categoryName];

            }

        }

    }

    return [];

}

model文件夹下的Topic.php

<?php

namespace app\index\model;

use think\Model;

class Topic extends Model{

    public static function getTopic($id) {

        return self::find(['id' => $id]);

    }

    public function user() {

        return $this->belongsTo('User', 'user_id');

    }

    /*public static function getTopic($id) {

        return self::withCount(['replies', 'praises'])-find($id);

    }*/

}