PHPunit example

src/Email.php

@@ -0,0 +1,33 @@
+<?php declare(strict_types=1);
+
+namespace MMK2410\MyTodoList;
+
+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)
+            );
+        }
+    }
+}

tests/EmailTest.php

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+<?php declare(strict_types=1);
+use PHPUnit\Framework\TestCase;
+use MMK2410\MyTodoList\Email;
+
+final class EmailTest extends TestCase
+{
+    private $email = 'user@example.com';
+
+    public function testCanBeCreatedFromValidEmailAddress(): void {
+        $this->assertInstanceOf(
+            Email::class,
+            Email::fromString($this->email)
+        );
+    }
+
+    public function testCannotBeCreatedFromInvalidEmailAddress(): void {
+        $this->expectException(InvalidArgumentException::class);
+        Email::fromString('invalid');
+    }
+
+    public function testCanBeUsedAsString(): void {
+        $this->assertEquals(
+            $this->email,
+            Email::fromString($this->email)
+        );
+    }
+}