I am trying to pass a variable to the callback function through add_meta_box. I know that there is $callback_args that can be used to pass the variable. But somehow I just can't get it to work within a class. This is my code:

class myclass{
  //...blah blah...adding action
  function add_ex_field() {
      __( 'My Custom Field', 'myplugin_textdomain'),
      //the $param is some value that is saved in my option table.
      array('test' => $param) 

  //do I need to include these two parameters? because they gave me error!
  function the_callback_function($post, $metabox){ 
    echo '<h1>testing field</h1>';
    echo $metabox['args']['test'];
global $myclass; 
$myclass = new myclass();



Sometimes an example says more than a thousand words. Here's a small plugin that will guide you:

! defined( 'ABSPATH' ) AND exit;
 * Plugin Name: (#65981) »kaiser« Meta Box example
 * Plugin URI:  http://goo.gl/ls6Q6
 * Description: Example showing how to add a meta box with callback args
 * Author:      Franz Josef Kaiser
 * Author URI:  http://unserkaiser.com

if ( ! class_exists( 'WPSE65981_MetaBox_Example' ) )
    add_action( 'load-post.php', array( 'WPSE65981_MetaBox_Example', 'init' ) );
    add_action( 'load-post-new.php', array( 'WPSE65981_MetaBox_Example', 'init' ) );

 * The Class
class WPSE65981_MetaBox_Example
    protected static $instance;

    public static function init()
        null === self :: $instance AND self :: $instance = new self;
        return self :: $instance;

    public function __construct()
        add_action( 'add_meta_boxes', array( $this, 'add_meta_box' ) );

     * Adds the meta box container
     * @return void
    public function add_meta_box()
            // ID
            // Title/"hndle"-bar
            ,__( 'WPSE Meta Box Headline', 'example_textdomain' )
            // Callback
            ,array( $this, 'render_meta_box_content' )
            // post_type
            // context
            // priority
            // callback_args
            ,array( 'Hello', "{$GLOBALS['current_user']->display_name}!" )

     * Render Meta Box content (Callback)
     * @return void
    public function render_meta_box_content( $post, $callback_args ) 
        // Argument dump: Uncomment for insights
        # var_dump( $post );
        # var_dump( $callback_args );

        $args = array_map( 'esc_html', $callback_args['args'] );
        return print "<h2>".implode( ' ', $args )."</h2>";
} // END Class WPSE65981_MetaBox_Example

} // endif;

If you're wondering, how I identified those hooks that are used for the static init function to load, then take a look at my »current admin info« plugin here.

enter image description here

  • 1
    You don't need to use &$this in add_(action|filter) calls now that we don't have to support PHP4 anymore. Objects that are stored in variables aren't actually stored in variables -- it's an identifier that the caller (call_user_func in this case) uses to look up the object. php.net/manual/en/language.oop5.references.php – chrisguitarguy Sep 22 '12 at 21:39
  • 1
    @chrisguitarguy Yes, I know. Thanks for the "heads up". I just took the (old) Codex example and modified it to fit the question. As you can see from the edits, I just overlooked the first one :) – kaiser Sep 22 '12 at 21:40
  • What if the variable that I want to pass is in an array? like this foreach($options['rules'] as $v){add_meta_box( 'pluginid', __( 'my plugin', 'myplugin_textdomain' ),array($this,'render_content'),$v['cpt'], array($v['variable']));} ? This give me Illegal offset type. The options is stored in the wp_option table. – dev-jim Sep 23 '12 at 8:37
  • @t.ck Your way of adding the var is not correct. Define the array before and just add the variable. $v['wrong']; vs. just $v. If you need to extract some part before, just do $n = $v['right'];. If you need to define an array, then just do it, save it in a var and add this var then as argument. – kaiser Sep 23 '12 at 8:40
  • I tried to define the var first like what you said.But I still get the error. – dev-jim Sep 23 '12 at 9:06

The simplest way to do this is, just remember the arguments being passed on a subset of the original array. While you are using a class structure, the basic non-class methodology still works.

So to pass an argument within the add_meta_box call, would be like this:

  $args = array('Index'=>'1');
  add_meta_box( 'mmd_listings_List_Group1',
           __( 'Title of Meta Box' ),
              'custom post name',
               $args    <========== array with arguments

 function MyCallBack($post_ID, $Arg)
  $Index = $Arg['args']['Index'];   <=========== your argument in the function

Check it out on: https://developer.wordpress.org/reference/functions/add_meta_box/

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