为什么这个PHP对象会将项目附加到自身？

I'm new to OOP and seem to be stuck on something that i'm sure is very simple, but I don't understand what's happening.

The below example code:

class add_some {

    static $arr = array('a', 'b');

    static $new_arr = array();

    public static function iterate() {

        foreach (self::$arr as $v) {
            self::$new_arr[] = $v;
        }

    }

}

for ($i=0; $i < 3; $i++) {

    // instantiate the object
    $add_some = new add_some;

    // launch the method that copies items from $arr to $new_arr
    $add_some::iterate();

    echo '<pre>';
    print_r($add_some::$new_arr);
    echo "</pre>";

    unset($add_some);

}

Goes through a loop and adds the items which are in $arr to $new_arr, it basically copies all items from $arr to $new_arr

The code instantiates a new object and executes the method iterate() which copies items from $arr to $new_arr 3 times.

Each iteration creates a new object $add_some = new add_some; so each iteration should just produce:

Array
(
    [0] => a
    [1] => b
Array
(
    [0] => a
    [1] => b
)
Array
(
    [0] => a
    [1] => b
)

But for some reason the object keeps growing with each iteration.... And I have no idea why.. I tried destroying the object (unset) and not sure if I should be doing something else.

Array
(
    [0] => a
    [1] => b
)
Array
(
    [0] => a
    [1] => b
    [2] => a
    [3] => b
)
Array
(
    [0] => a
    [1] => b
    [2] => a
    [3] => b
    [4] => a
    [5] => b
)

You are calling the static method by use of the :: (Paamayim Nekudotayim), you should be using -> and not using a static function.

Static means, keep this around globally, it is not instantiated each time you call new, it is static.

You can for example call a static method like so:

class MyClass {
  static function MyFunc() { echo 'Stuff'; }
}

MyClass::MyFunc()

There is no need to (nor should you) create an instance of MyClass to call the static MyFunc.

Each time you call iterate it is appending the global static array $arr

The correct way to do this would be

class add_some {
  private $x = 1;
  private $arr = array('a', 'b');
  public  $new_arr = array();

  public function iterate() {
    foreach ($this->arr as $v) {
        $this->new_arr[] = $v;
    }
  }
}

for ($i=0; $i < 3; $i++) {
  $add_some = new add_some();
  $add_some->iterate();
  print_r($add_some->new_arr);
}