Test run-time set of property specified in child class

ChildClass.php

<?php

class ChildClass extends ParentClass {

    protected $json;

    public function __construct() {
        parent::__construct();
    }

    public function toString() {
      return json_encode($this->json);
    }
}

index.php

<?php

require_once('ParentClass.php');
require_once('ChildClass.php');

$obj = new ChildClass();
echo $obj->toString();

ParentClass.php

<?php

class ParentClass {
    public function __construct() {
      // json is specified as a member but int he child class
      // will this cause an issue?
      $this->json = array("foo" => "bar");
    }
}