Initial project structure with Slim skeleton

src/Domain/User/User.php

@@ -0,0 +1,56 @@
+<?php
+declare(strict_types=1);
+
+namespace App\Domain\User;
+
+use JsonSerializable;
+
+class User implements JsonSerializable
+{
+    private ?int $id;
+
+    private string $username;
+
+    private string $firstName;
+
+    private string $lastName;
+
+    public function __construct(?int $id, string $username, string $firstName, string $lastName)
+    {
+        $this->id = $id;
+        $this->username = strtolower($username);
+        $this->firstName = ucfirst($firstName);
+        $this->lastName = ucfirst($lastName);
+    }
+
+    public function getId(): ?int
+    {
+        return $this->id;
+    }
+
+    public function getUsername(): string
+    {
+        return $this->username;
+    }
+
+    public function getFirstName(): string
+    {
+        return $this->firstName;
+    }
+
+    public function getLastName(): string
+    {
+        return $this->lastName;
+    }
+
+    #[\ReturnTypeWillChange]
+    public function jsonSerialize(): array
+    {
+        return [
+            'id' => $this->id,
+            'username' => $this->username,
+            'firstName' => $this->firstName,
+            'lastName' => $this->lastName,
+        ];
+    }
+}

src/Domain/User/UserNotFoundException.php

@@ -0,0 +1,11 @@
+<?php
+declare(strict_types=1);
+
+namespace App\Domain\User;
+
+use App\Domain\DomainException\DomainRecordNotFoundException;
+
+class UserNotFoundException extends DomainRecordNotFoundException
+{
+    public $message = 'The user you requested does not exist.';
+}

src/Domain/User/UserRepository.php

@@ -0,0 +1,19 @@
+<?php
+declare(strict_types=1);
+
+namespace App\Domain\User;
+
+interface UserRepository
+{
+    /**
+     * @return User[]
+     */
+    public function findAll(): array;
+
+    /**
+     * @param int $id
+     * @return User
+     * @throws UserNotFoundException
+     */
+    public function findUserOfId(int $id): User;
+}