StackOverflow closed my question http://anjane.sh/Z6G4YX while I got it answered at devnetwork http://anjane.sh/RxY6Vh

getter-setter

<?php
class A
{
    protected $_data;

    public function __get($name)
    {
        if (in_array($name, ['Firstname', 'Lastname']))
         return $this->_data[$name];            
    }   
    
    public function __set($name, $value)
    {        
        if (in_array($name, ['Firstname', 'Lastname']))
        {
            $this->_data[$name] = $value;
            return;
        }        
    }   
}
class B extends A
{
    public function __get($name)
    {
        $value = parent::__get($name);        
        # echo gettype($value)."\n";
        if ($value) return $value; # If not set, $value would be NULL
        
        if (in_array($name, ['City', 'Zipcode']))
         return $this->_data[$name];                
    }
    
    public function __set($name, $value)
    {
        parent::__set($name, $value);
        if (in_array($name, ['City', 'Zipcode']))
        {
            $this->_data[$name] = $value;
            return;
        }
    }   
}

$b = new B();

$b->Firstname = "John";
$b->Lastname  = "Smith";
$b->City      = "London";
$b->Zipcode   = "W11 2BQ";

echo $b->Firstname."\n";
echo $b->Lastname."\n";
echo $b->City."\n";
echo $b->Zipcode."\n";
?>