THE SOLUTION

As explained here latest version of laravel 6, 7 and 8 has made changes required for php 8. all you have to do is:

1- add php 8 to your composer.json (I've kept v7.4 just in case production server does not support php 8 yet)

"php": "^7.4|^8.0",

2- to run composer update to update your laravel to the latest version

composer update

3- make sure update the following libraries since they exist in all laravel applications

PHP to php:^8.0
Faker to fakerphp/faker:^1.9.1
PHPUnit to phpunit/phpunit:^9.3

4- check for any other library which needs to be updated, contribute if they haven't supported php 8. but you should be good to go with most of the libraries since they have active contributors.

EXPLAINING THE PROBLEM

as described here

PHP 8 introduces several improvements in PHP type systems such as the introduction of Union Types, mixed type, and a few more.

With these changes, certain methods in Reflection API's ReflectionParameter yield incorrect results.

In PHP 8, the following methods from ReflectionParameter class is deprecated:

ReflectionParameter::getClass()
ReflectionParameter::isArray()
ReflectionParameter::isCallable()

ReflectionParamter::getType() is the recommended way to replace the deprecated methods. This method is available in PHP 7.0 and later.

I had similar issue. But I had already run brew update and brew cleanup before I noticed the issue. What I did:

1. I noticed this error from brew cleanup:

Error: The `brew link` step did not complete successfully
The formula built, but is not symlinked into /usr/local
Could not symlink bin/node
Target /usr/local/bin/node
already exists. You may want to remove it:
  rm '/usr/local/bin/node'

To force the link and overwrite all conflicting files:
  brew link --overwrite heroku-node

2. I ran the commands:

brew link --overwrite composer
composer upgrade
composer update

That's what worked for me

Check your php version in your virtual machine(xampp or server).

php --version

Is that version PHP 8 ? Am I right? That's the cause of the problem:

PHP 8 introduces several improvements in PHP type systems such as the introduction of Union Types, mixed type, and a few more.

With these changes, certain methods in Reflection API's ReflectionParameter yield incorrect results.

In PHP 8, the following methods from ReflectionParameter class is deprecated:

• ReflectionParameter::getClass()
• ReflectionParameter::isArray()
• ReflectionParameter::isCallable()
• ReflectionParamter::getType()

Downgrade your php version to 7.4 and your Laravel app works like a charm!