Address dynamic class property deprecation warnings

models/Guest.php

@@ -21,7 +21,7 @@ class Guest extends User
 		$this->is_guest = true;
 		$this->is_admin = false;
 		$this->first_name = 'Guest';
-		$this->last_name = '';
+		$this->surname = '';
 	}
 
 	public function updateAccessTime()