Challenge 2 — Solution

Task: Write a composer.json defining a PSR-4 mapping for the namespace
prefix "App\\" to a folder "app/". Describe in a comment which command
would be run after creating/editing this file, and what it generates.

---- composer.json ----

{
    "autoload": {
        "psr-4": {
            "App\\": "app/"
        }
    }
}

---- Notes (as would be written in a comment) ----

After creating or editing composer.json, the command to run is:

    composer dump-autoload

This regenerates vendor/autoload.php - a single generated file that
knows how to map any class name starting with "App\" (e.g. App\Models\
User, App\Services\EmailService) directly onto a file path under the
app/ folder (app/Models/User.php, app/Services/EmailService.php),
following the PSR-4 convention. Any PHP script can then get access to
every class under the App\ namespace just by writing:

    require 'vendor/autoload.php';

at the top, with no manual require_once needed for any individual
class file from that point onward.

Notes:
- The double backslash "App\\" in the JSON is required because a
  single backslash is JSON's own escape character - "App\\" represents
  the single literal backslash PHP actually uses as its namespace
  separator.
- "app/" (with the trailing slash) tells Composer's autoloader exactly
  which folder on disk corresponds to the App\ namespace prefix - a
  class App\Models\User would be expected at app/Models/User.php.
- composer dump-autoload only needs to be re-run when the autoload
  mapping itself changes (a new namespace prefix, a renamed folder) -
  adding a new class file within an already-mapped namespace/folder
  works immediately without re-running it, since PSR-4 computes the
  path from the class name rather than listing every file explicitly.