下列的this tutorial, 我试图通过向function.php 文件：

add_meta_box( \'my-meta-box-id\', \'My First Meta Box\', \'cd_meta_box_cb\', \'post\', \'normal\', \'high\' );
add_action( \'add_meta_boxes\', \'cd_meta_box_add\' );

function cd_meta_box_add()
{
  add_meta_box( \'my-meta-box-id\', \'My First Meta Box\', \'cd_meta_box_cb\', \'post\', \'normal\', \'high\' );
}

function cd_meta_box_cb()  
{  
  echo \'What you put here, show\\\'s up in the meta box\';     
}  

但我发现以下错误：

( ! ) Fatal error: Call to undefined function add_meta_box() in C:\\wamp\\www\\TMPress\\wp-content\\themes\\twentytwelve\\cpt.php on line 94
Call Stack
#   Time    Memory  Function    Location
1   0.0012  865440  {main}( )   ..\\edit.php:0
2   0.0018  982824  require_once( \'C:\\wamp\\www\\TMPress\\wp-admin\\admin.php\' )    ..\\edit.php:10
3   0.0021  1008744 require_once( \'C:\\wamp\\www\\TMPress\\wp-load.php\' )   ..\\admin.php:30
4   0.0023  1026872 require_once( \'C:\\wamp\\www\\TMPress\\wp-config.php\' ) ..\\wp-load.php:29
5   0.0029  1158248 require_once( \'C:\\wamp\\www\\TMPress\\wp-settings.php\' )   ..\\wp-config.php:90
6   0.3251  32951888    include( \'C:\\wamp\\www\\TMPress\\wp-content\\themes\\twentytwelve\\functions.php\' )   ..\\wp-settings.php:291
7   0.3255  32979176    include_once( \'C:\\wamp\\www\\TMPress\\wp-content\\themes\\twentytwelve\\cpt.php\' )    ..\\functions.php:2

你能告诉我为什么会这样吗？有没有更好的方法添加包含字段和下拉选择框的元框？我知道有一些插件可以做这件事，但我真的很喜欢自己学习。

add_action( \'add_meta_boxes\', \'cd_meta_box_add\' );

function cd_meta_box_add()
{
  add_meta_box( \'my-meta-box-id\', \'My First Meta Box\', \'cd_meta_box_cb\', \'post\', \'normal\', \'high\' );
}

function cd_meta_box_cb()  
{  
  echo \'What you put here, show\\\'s up in the meta box\';     
}  

add_meta_box( \'my-meta-box-id\', \'My First Meta Box\', \'cd_meta_box_cb\', \'post\', \'normal\', \'high\' );

add_meta_box( \'my-meta-box-id\', \'My First Meta Box\', \'cd_meta_box_cb\', \'your-custom-post-type\', \'normal\', \'high\' );