吃水不忘挖井人，thanks to Eric Don

首先，Laravel数据库配置信息配置文件优先级：.env > config/database.php

• 单个数据库配置(应用默认数据库配置)一般是配置在.env文件中即可
• 多个数据库配置(额外的数据库配置)一般是配置在config/database.php文件中

config/database.php默认配置如下(因版本不同可能内容存在差异)：

<?php

return [

    /*
    |--------------------------------------------------------------------------
    | Default Database Connection Name
    |--------------------------------------------------------------------------
    |
    | Here you may specify which of the database connections below you wish
    | to use as your default connection for all database work. Of course
    | you may use many connections at once using the Database library.
    |
    */

    'default' => env('DB_CONNECTION', 'mysql'),

    /*
    |--------------------------------------------------------------------------
    | Database Connections
    |--------------------------------------------------------------------------
    |
    | Here are each of the database connections setup for your application.
    | Of course, examples of configuring each database platform that is
    | supported by Laravel is shown below to make development simple.
    |
    |
    | All database work in Laravel is done through the PHP PDO facilities
    | so make sure you have the driver for your particular database of
    | choice installed on your machine before you begin development.
    |
    */

    'connections' => [

        'sqlite' => [
            'driver' => 'sqlite',
            'database' => env('DB_DATABASE', database_path('database.sqlite')),
            'prefix' => '',
        ],

        'mysql' => [
            'driver' => 'mysql',
            'host' => env('DB_HOST', '127.0.0.1'),
            'port' => env('DB_PORT', '3306'),
            'database' => env('DB_DATABASE', 'forge'),
            'username' => env('DB_USERNAME', 'forge'),
            'password' => env('DB_PASSWORD', ''),
            'unix_socket' => env('DB_SOCKET', ''),
            'charset' => 'utf8mb4',
            'collation' => 'utf8mb4_unicode_ci',
            'prefix' => '',
            'strict' => true,
            'engine' => null,
        ],

        'pgsql' => [
            'driver' => 'pgsql',
            'host' => env('DB_HOST', '127.0.0.1'),
            'port' => env('DB_PORT', '5432'),
            'database' => env('DB_DATABASE', 'forge'),
            'username' => env('DB_USERNAME', 'forge'),
            'password' => env('DB_PASSWORD', ''),
            'charset' => 'utf8',
            'prefix' => '',
            'schema' => 'public',
            'sslmode' => 'prefer',
        ],

        'sqlsrv' => [
            'driver' => 'sqlsrv',
            'host' => env('DB_HOST', 'localhost'),
            'port' => env('DB_PORT', '1433'),
            'database' => env('DB_DATABASE', 'forge'),
            'username' => env('DB_USERNAME', 'forge'),
            'password' => env('DB_PASSWORD', ''),
            'charset' => 'utf8',
            'prefix' => '',
        ],

    ],

    /*
    |--------------------------------------------------------------------------
    | Migration Repository Table
    |--------------------------------------------------------------------------
    |
    | This table keeps track of all the migrations that have already run for
    | your application. Using this information, we can determine which of
    | the migrations on disk haven't actually been run in the database.
    |
    */

    'migrations' => 'migrations',

    /*
    |--------------------------------------------------------------------------
    | Redis Databases
    |--------------------------------------------------------------------------
    |
    | Redis is an open source, fast, and advanced key-value store that also
    | provides a richer set of commands than a typical key-value systems
    | such as APC or Memcached. Laravel makes it easy to dig right in.
    |
    */

    'redis' => [

        'client' => 'predis',

        'default' => [
            'host' => env('REDIS_HOST', '127.0.0.1'),
            'password' => env('REDIS_PASSWORD', null),
            'port' => env('REDIS_PORT', 6379),
            'database' => 0,
        ],

    ],

];

官方相关文档：https://laravel.com/docs/5.5/database#using-multiple-database-connections

因此，动态修改config/database.php中的配置即可实现预期效果。

我们可以在应用启动的Provider中进行配置，官方相关文档：https://laravel.com/docs/5.5/providers

在app/Providers/AppServiceProvider.php中的boot方法中加入如下代码：

//connections
if (\Schema::hasTable('connections')) {
        $connections = \App\Models\Connection::all();
        $db = \Config::get('database.connections');
        foreach ($connections as $connection)
        {
            $db['cdb_'.$connection->id] = [
                    'driver'    => 'mysql',
                    'host'      => $connection->db_host,
                    'port'      => $connection->db_port,
                    'database'  => $connection->db_database,
                    'username'  => $connection->db_user,
                    'password'  => $connection->db_pass,
                    'charset'   => 'utf8',
                    'collation' => 'utf8_unicode_ci', //utf8_unicode_ci
                    'prefix'    => '',
                    'strict'    => false,
        ];
    }
    \Config::set('database.connections',$db);
}else{
    //\Config::set('database.connections',[]);
}
            

使用：

\DB::connection("cdb_1")->table("table_name")->...

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