PHPUnit Class not found error with Composer

Question

I'm new to PHPUnit and I can't get it working

I install PHPUnit with Composer using this command:

C:\xampp\htdocs\unit> composer require --dev phpunit/phpunit ^9
C:\xampp\htdocs\unit> .\vendor\bin\phpunit --version
PHPUnit 9.0.0 by Sebastian Bergmann and contributors.

I then create a src folder where I put my Email.php:

<?php declare(strict_types=1);
final class Email
{
    private $email;

    private function __construct(string $email)
    {
        $this->ensureIsValidEmail($email);

        $this->email = $email;
    }

    public static function fromString(string $email): self
    {
        return new self($email);
    }

    public function __toString(): string
    {
        return $this->email;
    }

    private function ensureIsValidEmail(string $email): void
    {
        if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
            throw new InvalidArgumentException(
                sprintf(
                    '"%s" is not a valid email address',
                    $email
                )
            );
        }
    }
}

And a tests folder where I create EmailTest.php:

<?php declare(strict_types=1);
use PHPUnit\Framework\TestCase;

final class EmailTest extends TestCase
{
    public function testCanBeCreatedFromValidEmailAddress(): void
    {
        $this->assertInstanceOf(
            Email::class,
            Email::fromString('user@example.com')
        );
    }

    public function testCannotBeCreatedFromInvalidEmailAddress(): void
    {
        $this->expectException(InvalidArgumentException::class);

        Email::fromString('invalid');
    }

    public function testCanBeUsedAsString(): void
    {
        $this->assertEquals(
            'user@example.com',
            Email::fromString('user@example.com')
        );
    }
}

When I try to run the tests I get the following error:

    C:\xampp\htdocs\unit>.\vendor\bin\phpunit tests
PHPUnit 9.0.0 by Sebastian Bergmann and contributors.

EFE                                                                 3 / 3 (100%)

Time: 00:02.229, Memory: 6.00 MB

There were 2 errors:

1) EmailTest::testCanBeCreatedFromValidEmailAddress
Error: Class 'Email' not found

C:\xampp\htdocs\unit\tests\EmailTest.php:10

2) EmailTest::testCanBeUsedAsString
Error: Class 'Email' not found

C:\xampp\htdocs\unit\tests\EmailTest.php:25

--

There was 1 failure:

1) EmailTest::testCannotBeCreatedFromInvalidEmailAddress
Failed asserting that exception of type "Error" matches expected exception "InvalidArgumentException". Message was: "Class 'Email' not found" at
C:\xampp\htdocs\unit\tests\EmailTest.php:18
.

ERRORS!
Tests: 3, Assertions: 1, Errors: 2, Failures: 1.

It seems that the Email class can't be found, even though it exists in the src folder.

My project structure looks like this:

-src
-tests
-vendor
-composer.json
-composer.lock

My composer.json looks like this:

{
    "require-dev": {
        "phpunit / phpunit": "^ 8"
    }
}

And here's my autoload.php:

<?php

// autoload.php @generated by Composer

require_once __DIR__ . '/composer/autoload_real.php';

return ComposerAutoloaderInit404f8346552faf7774cab6726655b430::getLoader();

Could anyone please show how to fix this issue and get PHPUnit working?