The application object $app in laravel is written as $app[$k] but no error is reported! Why?

In the getConfig
method in the IlluminateFilesystemFilesystemManager
class in laravel 5.1,
actually uses

<code class="php">$this->app['config']["filesystems.disks.{$name}"]);</code>

Return array.

But

<code class="php">$this->app</code>

Obviously a target.
Can an object be retrieved using the key value of an array? This is obviously grammatically wrong but something magical happened anyway

This is the getConfig method

<code class="php"> /**
     * Get the filesystem connection configuration.
     *
     * @param  string  $name
     * @return array
     */
    protected function getConfig($name)
    {
       
        return $this->app['config']["filesystems.disks.{$name}"];
    }</code>

I alone dd($this->app);
That is as follows

<code class="php"> /**
     * Get the filesystem connection configuration.
     *
     * @param  string  $name
     * @return array
     */
    protected function getConfig($name)
    {
        dd($this->app);
        return $this->app['config']["filesystems.disks.{$name}"];
    }</code>

Output

But I dd($this->app'config'); which is

<code class="php"> protected function getConfig($name)
    {
        dd($this->app['config']["filesystems.disks.{$name}"]);
        return $this->app['config']["filesystems.disks.{$name}"];
    }</code>

Then the output is as follows

In short, $app is obviously an object, how can it be written in the form $app[$k]?

Reply content:

In the getConfig
method in the IlluminateFilesystemFilesystemManager
class in laravel 5.1,
actually uses

<code class="php">$this->app['config']["filesystems.disks.{$name}"]);</code>

Return array.

But

<code class="php">$this->app</code>

Obviously a target.
Can an object be retrieved using the key value of an array? This is obviously grammatically wrong but something magical happened anyway

This is the getConfig method

<code class="php"> /**
     * Get the filesystem connection configuration.
     *
     * @param  string  $name
     * @return array
     */
    protected function getConfig($name)
    {
       
        return $this->app['config']["filesystems.disks.{$name}"];
    }</code>

I alone dd($this->app);
That is as follows

<code class="php"> /**
     * Get the filesystem connection configuration.
     *
     * @param  string  $name
     * @return array
     */
    protected function getConfig($name)
    {
        dd($this->app);
        return $this->app['config']["filesystems.disks.{$name}"];
    }</code>

Output

But I dd($this->app'config'); which is

<code class="php"> protected function getConfig($name)
    {
        dd($this->app['config']["filesystems.disks.{$name}"]);
        return $this->app['config']["filesystems.disks.{$name}"];
    }</code>

Then the output is as follows

In short, $app is obviously an object, how can it be written in the form $app[$k]?

app inherits from IlluminateContainerContainer, and Container implements the ArrayAccess (http://php.net/manual/zh/clas...) interface. The ArrayAccess interface provides the ability to access objects just like accessing arrays. As long as you implement a few methods of the interface, you can call isset, unset, [] to access values.

$this->app['config'] is also an object IlluminateConfigRepository It also implements ArrayAccess, so it can also be used as an array.

ArrayAccess