Question

I’m in the process of writing a WordPress plugin which creates a page in the admin area, as well as executing some frontend code.

The code below throws a nice Fatal error: Using $this when not in object context error. Which is rather mystifying, as the variable is called inside the class.

Maybe I’m not following the intended WordPress plugin structure for functions and classes, but the conceptual code below was created using the relevant entries on plugin development in the WordPress Codex.

Could somebody explain why the error is triggered, because when I create an instance of the class outside of the WordPress codebase everything is fine.

if (!class_exists("MyClass")) {
  class MyClass {
    var $test = 'Test variable';

    public function index() {
      //Index code
    }

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

add_action('admin_menu', 'my_plugin_menu');

function my_plugin_menu() {
  add_menu_page('My Plugin', 'My Plugin', 'manage_options', 'my-plugin', array('MyClass', 'index'));
  add_submenu_page('my-plugin', 'Add New Thing', 'Add New', 'manage_options', 'my-plugin-add', array('MyClass', 'add'));
}

Answer 1

So, I’ve seem to have fixed it, by going back to the basics and asking Google the humble question: “Using classes in WordPress plugins”.

Both the article by Jay Fortner and one on dConstructing.com were helpful.

Basically, I’m now calling add_menu_page and add_submenu_page from within the class. I was under the impression those functions somehow created an object, but they obviously don’t.

My code now looks something like this and I’m able to call the declared class variable without error:

if (!class_exists("MyClass")) {
  class MyClass {
    var $test = 'Test variable';

    function __construct() {
      add_action('admin_menu', 'my_plugin_menu');
    }

    function my_plugin_menu() {
      add_menu_page('My Plugin', 'My Plugin', 'manage_options', 'my-plugin', array(&$this, 'index'));
      add_submenu_page('my-plugin', 'Add New Thing', 'Add New', 'manage_options', 'my-plugin-add', array(&$this, 'add'));
    }

    public function index() {
      //Index code
    }

    public function add() {
      echo $this->test;
    }
  }
  new MyClass;
}