ある日、PHPのLaravel入門の本(通称青本)をもとに環境構築しているときのことでした。
php artisan serveを叩いたところ、こんなエラーメッセージが、、
※DeskTopにlaravelappという名前でプロジェクト作成しています。

PHP Warning:  require(C:\Users\ユーザー名\Desktop\laravelapp/vendor/autoload.php): Failed to open stream: No such file or directory in C:\Users\ユーザー名\Desktop\laravelapp\artisan on line 18
PHP Fatal error:  Uncaught Error: Failed opening required 'C:\Users\ユーザー名\Desktop\laravelapp/vendor/autoload.php' (include_path='.;C:\php\pear') in C:\Users\ユーザー名\Desktop\laravelapp\artisan:18
Stack trace:
#0 {main}
  thrown in C:\Users\ユーザー名\Desktop\laravelapp\artisan on line 18

調べたところ、 composer installや、 composer updateを叩くとautoload.phpが落とせるらしい。。

composer installをたたてみた。
ムムム。。。。

No composer.lock file present. Updating dependencies to latest instead of installing from lock file. See https://getcomposer.org/install for more information.
Loading composer repositories with package information
Info from https://repo.packagist.org: #StandWithUkraine
Updating dependencies
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - league/mime-type-detection[1.0.0, ..., 1.3.0] require php ^7.2 -> your php version (8.2.3) does not satisfy that requirement.
    - league/mime-type-detection[1.4.0, ..., 1.11.0] require ext-fileinfo * -> it is missing from your system. Install or enable PHP's fileinfo extension.
    - laravel/framework v10.0.0 requires league/flysystem ^3.8.0 -> satisfiable by league/flysystem[3.8.0, ..., 3.12.2].
    - league/flysystem[3.8.0, ..., 3.12.2] require league/mime-type-detection ^1.0.0 -> satisfiable by league/mime-type-detection[1.0.0, ..., 1.11.0].
    - Root composer.json requires laravel/framework ^10.0 -> satisfiable by laravel/framework[v10.0.0].

To enable extensions, verify that they are enabled in your .ini files:
    - C:\Program Files\php-8.2\php.ini
You can also run `php --ini` in a terminal to see which files are used by PHP in CLI mode.
Alternatively, you can run Composer with `--ignore-platform-req=ext-fileinfo` to temporarily ignore these required extensions.

composer updateも同じメッセージだった。。　見てみると、
you can run Composer with `--ignore-platform-req=ext-fileinfo` to temporarily ignore these required extensions.

とかいてありますね。

composer update --ignore-platform-req=ext-fileinfo

これをたたいてみます。
おおお動いた

Loading composer repositories with package information
Updating dependencies
Lock file operations: 106 installs, 0 updates, 0 removals
  - Locking brick/math (0.10.2)
  - Locking dflydev/dot-access-data (v3.0.2)
  - Locking doctrine/inflector (2.0.6)
  - Locking doctrine/lexer (3.0.0)
  - Locking dragonmantank/cron-expression (v3.3.2)
  - Locking egulias/email-validator (4.0.1)
  - Locking fakerphp/faker (v1.21.0)
  - Locking filp/whoops (2.14.6)
  - Locking fruitcake/php-cors (v1.2.0)
  - Locking graham-campbell/result-type (v1.1.0)
  - Locking guzzlehttp/guzzle (7.5.0)
  - Locking guzzlehttp/promises (1.5.2)
  - Locking guzzlehttp/psr7 (2.4.3)
====================以下略=================

結構長かったです。5分くらいかな。
php artisan serveをしたら無事起動できました。