PHP 추상 클래스가 추상 클래스를 상속할 때 주의할 점

<?
abstract class User 
{
    protected $sal = 0;  
  
    abstract function getSal();
    abstract function setSal($sal);  
}
abstract class VipUser extends User {
     
}
?>

<?
abstract class User 
{
    protected  $sal = 0;  
  
    abstract function getSal();
    abstract function setSal($sal);
}
abstract class VipUser extends User 
{
    abstract function setSal();
}
?>

Fatal error: Can't inherit abstract function User::setSal() (previously declared abstract in VipUser) in E:\PH

<?
abstract class User 
{
    protected $sal = 0;  
  
    abstract function getSal();
    abstract function setSal($sal);
}
abstract class VipUser extends User 
{
    protected $commision = 0;
    static abstract function getCommision();
    abstract function setCommision();   
}
?>