Update User.php

php8.1 test error handle

Author: hawk2020

Behavioral/Observer/User.php

@@ -22,23 +22,23 @@ public function __construct()
         $this->observers = new SplObjectStorage();
     }
 
-    public function attach(SplObserver $observer)
+    public function attach(SplObserver $observer): void
     {
         $this->observers->attach($observer);
     }
 
-    public function detach(SplObserver $observer)
+    public function detach(SplObserver $observer): void
     {
         $this->observers->detach($observer);
     }
 
-    public function changeEmail(string $email)
+    public function changeEmail(string $email): void
     {
         $this->email = $email;
         $this->notify();
     }
 
-    public function notify()
+    public function notify(): void
     {
         /** @var SplObserver $observer */
         foreach ($this->observers as $observer) {