How to pass variable via $callback_args for add_meta_box

Question

I trying to pass some variable to the callback function through add_meta_box. Knowing that there is $callback_args that can be use to pass the variable. But somehow I just can't get it work within a class. This is my codes

class myclass{
 //.....blah blah....adding action
  function add_ex_field() {

add_meta_box( 'apextadmin', __( 'My Custom Field', 'myplugin_textdomain' ),array($this,'the_callback_function'),post,array('test'=> $param));}//the $param is some value that saved in my option table.
}

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

}
global $myclas; 
$myclas = new myclas();

Thanks.

Answer 1

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

<?php
! 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()
    {
        add_meta_box(
            // ID
             'wpse65981_meta_box_example'
            // Title/"hndle"-bar
            ,__( 'WPSE Meta Box Headline', 'example_textdomain' )
            // Callback
            ,array( $this, 'render_meta_box_content' )
            // post_type
            ,'post'
            // context
            ,'advanced'
            // priority
            ,'high'
            // callback_args
            ,array( 'Hello', "{$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.

Answer 2

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' ),
              'MyCallBack',
              'custom post name',
              'normal',
              'high',
               $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/