如何创建仅继承方法而不继承父属性的子类？

Question

Suppose I have this code:

First I created the Parent

 class Country {
  public $country;
  public $populationcountry;
  public $language;     

with some methods(which are not relevant for this question)

public function function1() {   }
public function function2() {    }
             .
             .
                }  //end of parent class

Then I create the child

 Class city extends Country 
{
public $populationcity;
} 

Then I create the objects (fot this example I only created one)

$city1 = new city();

$city1->populationcity = 10000; 

and an array of the objects

$cities = [$city1];    

And finally I want to 'echo' only the child's properties (populationcity)

foreach ($cities as $city) {
foreach ($city as $k => $v) {
$city->populationcity;
echo $k . ': ' . $v . '<br>';
 }
}   

Output :
populationcity:10000
country:
populationcountry:
language:

I want to keep the methods of the parent, but not the properties of the parent. How can I achieve that?

---

David in the comments told me to set the properties to Private. I did so, and it worked fine but when I created a Country object , it prints the parent's property in the child class.

This is the code, which gives me this output when the parent properties are Public.

populationcity: 10000
country:
populationcountry:
language:
country: England
populationcountry: 30000
language: English

It should print:

populationcity: 10000
country: England
populationcountry: 30000
language: English

When I set them to Private I get this:

Fatal error: Uncaught Error: Cannot access private property Country::$language in C:\\xampp\\htdocs\\ejercicios.php:141 Stack trace: #0 {main} thrown in C:\\xampp\\htdocs\\ejercicios.php on line 141

And when I set them to Protected I get this:

Fatal error: Uncaught Error: Cannot access protected property Country::$language in C:\\xampp\\htdocs\\ejercicios.php:141 Stack trace: #0 {main} thrown in C:\\xampp\\htdocs\\ejercicios.php on line 141

class Country {
  public $country;
  public $populationcountry;
  public $language;
  }

Class city extends Country 
{
public $populationcity;
}

$country1 = new Country();
$country1->language = 'English';
$country1->country = 'ENgland';
$country1->populationcountry = 30000;
$countries = [$country1];

$city1 = new city();
$city1->populationcity = 10000;

$cities = [$city1];

foreach ($cities as $city) {
 foreach ($city as $k => $v) {
$city->populationcity;
echo $k . ': ' . $v . '<br>';
}
}

foreach ($countries as $country) {
foreach ($country as $k => $v) {
$country->language;
$country->populationcountry;
$country->country; 
echo $k . ': ' . $v . '<br>';

}

}

Answer 1

Looks like a bad design pattern. For me a country can contain several different cities. Therefore a city is not a country. Shouldn 't your country entity class look like this?

class Country
{
    /**
     * Collection of City objects
     * @var ArrayObject
     */
    protected $cities;
    protected $name;
    protected $population;
    protected $language;

    public function setCity(City $city) : self
    {
        if ($this->cities == null) {
            $this->cities = new ArrayObject();
        }

        $this->cities->append($city);
        return $this;
    }

    public function getCities() : ArrayObject
    {
        if ($this->cities == null) {
            $this->cities = new ArrayObject();
        }

        return $this->cities;
    }
}

Anyway ... let us solve your issue with php's own reflection classes. To get the properties of the declaring class only just implement the following function in the city class.

class City extends Country
{
    public $populationCity;

    public function getData() : array
    {
        $data = [];
        $reflection = new ReflectionClass($this);
        $properties = $reflection->getProperties(ReflectionProperty::IS_PUBLIC);

        foreach ($properties as $property) {
            if ($property->getDeclaringClass() == $reflection) {
                $data[$property->getName()] = $property->getValue($this);
            }
        }

        return $data;
    }
}

With this you can get the properties of the City class only. Let us have a try ...

$city = new City();
$city->populationCity = 10000;

var_dump($city->getData());

Hope this helps.

Answer 2

You can use Reflection to achieve this.

Reflection API that adds the ability to reverse-engineer classes, interfaces, functions, methods and extensions. Additionally, the reflection API offers ways to retrieve doc comments for functions, classes and methods.

This will allow you to enumerate the properties and see if any given one is defined in the class itself or along its inheritance chain.

foreach ($cities as $city) {
    /* You can move this logic out of the loop and cache the properties 
       if your array contains only elements of the same type */
    $reflector = new ReflectionClass($city);
    $class = $reflector->getName(); // Get the name of the current class

    $ownProperties = array_map(function ($e) { return $e->name; },  // Return only the name of 
    // the property
        array_filter($reflector->getProperties(), // Return properties defined in 
        // the same class only, regardless the visibility, since protected properties can
        // either be declared or inherited
             // Check if declaring class is the same as this type
            function($e) use ($class) { return $class === $e->class; } ));
      /* **** */
    foreach ($ownProperties as $property) {
        echo $property . ': ' . $city->$property . '<br>';
    }
} 

Demo