PHPに属性を追加する方法

<?php
error_reporting(-1);
ini_set(&#39;display_errors&#39;,&#39;on&#39;);
 
class A { 
    public $a = &#39;hello&#39;;
 
    public function add() {
        $this->b = &#39;world&#39;;
    }-  
    public static function p() {
        echo &#39;world&#39;,PHP_EOL;
    }-  
}
 
$a = new A;
$a->add();
$a->c = &#39;test&#39;;
$a->p();
 
var_dump($a);

world
object(A)#1 (3) {
  ["a"]=>
  string(5) "hello"
  ["b"]=>
  string(5) "world"
  ["c"]=>
  string(4) "test"
}

$a->p();