Add method 'registerWithUniqueUsername'

README.md

@@ -105,6 +105,8 @@ catch (\Delight\Auth\TooManyRequestsException $e) {
 
 The username in the third parameter is optional. You can pass `null` here if you don't want to manage usernames.
 
+If you want to enforce unique usernames, on the other hand, simply call `registerWithUniqueUsername` instead of `register`, and be prepared to catch the `DuplicateUsernameException`, if necessary.
+
 For email verification, you should build an URL with the selector and token and send it to the user, e.g.:
 
 ```php

src/Auth.php

@@ -177,6 +177,36 @@ class Auth {
 		return $this->createUserInternal(false, $email, $password, $username, $callback);
 	}
 
+	/**
+	 * Attempts to sign up a user while ensuring that the username is unique
+	 *
+	 * If you want the user's account to be activated by default, pass `null` as the callback
+	 *
+	 * If you want to make the user verify their email address first, pass an anonymous function as the callback
+	 *
+	 * The callback function must have the following signature:
+	 *
+	 * `function ($selector, $token)`
+	 *
+	 * Both pieces of information must be sent to the user, usually embedded in a link
+	 *
+	 * When the user wants to verify their email address as a next step, both pieces will be required again
+	 *
+	 * @param string $email the email address to register
+	 * @param string $password the password for the new account
+	 * @param string|null $username (optional) the username that will be displayed
+	 * @param callable|null $callback (optional) the function that sends the confirmation email to the user
+	 * @return int the ID of the user that has been created (if any)
+	 * @throws InvalidEmailException if the email address was invalid
+	 * @throws InvalidPasswordException if the password was invalid
+	 * @throws UserAlreadyExistsException if a user with the specified email address already exists
+	 * @throws DuplicateUsernameException if the specified username wasn't unique
+	 * @throws AuthError if an internal problem occurred (do *not* catch)
+	 */
+	public function registerWithUniqueUsername($email, $password, $username = null, callable $callback = null) {
+		return $this->createUserInternal(true, $email, $password, $username, $callback);
+	}
+
 	/**
 	 * Creates a request for email confirmation
 	 *