Plugin code will not work properly inside a class [closed]

by

Enable the magic method __construct

You have to change the following line

$MySubBoxClass = new MySubBoxClass;

Into

$MySubBoxClass = new MySubBoxClass();

This way the __construct() magic method will be used.
The add_action methods aren’t called now.

Error in html_form_func( $object )

Change

public function html_form_func( $object ) //Creates the HTML form and outputs its value if it has one   
{  
    wp_nonce_field( basename( __FILE__ ), 'meta_box_nonce' );

    <label><input type="text" name="subheading" id="meta-box-input" size="144" value="<?php echo get_post_meta( $object->ID, 'subheading', true ); ?>" /></label>        
}

Into

public function html_form_func( $object ) //Creates the HTML form and outputs its value if it has one   
{  
    wp_nonce_field( basename( __FILE__ ), 'meta_box_nonce' );
    ?>
    <label><input type="text" name="subheading" id="meta-box-input" size="144" value="<?php echo get_post_meta( $object->ID, 'subheading', true ); ?>" /></label>
    <?php
}

Object methods by reference

Change

function __construct()
  {
  add_action( 'add_meta_boxes', array( $this, 'meta_box_add' ) );      //Hooks meta_box_add() onto the add_meta_boxes hook                                                                

  add_action( 'save_post', array( $this, 'save_meta_box', 10, 2 ) );   //Hooks save_meta_box() onto the save_post hook                                                                          

  add_action('admin_init', array( $this, 'remove_custom_meta_boxes') );//Hooks remove_custom_meta_boxes() onto admin_init  
  }

Into

function __construct()
{
    add_action( 'add_meta_boxes', array( &$this, 'meta_box_add' ) );        //Hooks meta_box_add() onto the add_meta_boxes hook                                                                
    add_action( 'save_post', array( &$this, 'save_meta_box', 10, 2 ) );     //Hooks save_meta_box() onto the save_post hook                                                                          
    add_action( 'admin_init', array( &$this, 'remove_custom_meta_boxes' ) );//Hooks remove_custom_meta_boxes() onto admin_init  
}

If you use object methods in add_action you have to use &$this to use $this in reference.