为什么migration_path需要可写？不应该php只需要读取文件？我查看了系统文件夹中的迁移代码，找不到原因。
以下是application / config / migration.php
中的注释/*
|--------------------------------------------------------------------------
| Migrations Path
|--------------------------------------------------------------------------
|
| Path to your migrations folder.
| Typically, it will be within your application path.
| Also, writing permission is required within the migrations path.
|
*/
$config['migration_path'] = APPPATH.'migrations/';