class A {
  public $item_limit;

   public function __construct(){
        $this->item_limit = null;
   }

  function test() {
    echo $this->item_limit;
  }

}

class B extends A {

   function My_test() {
        $tools = new A();
        if (isset($limit))
             $tools->item_limit = $limit;
   }
}