/* Define the custom Box，适用WP 3.0以后的版本 */
add_action( 'add_Meta_Boxes','myplugin_add_custom_Box' );

// 如果是WP 3.0之前的版本，使用以下一行代码
// add_action( 'admin_init','myplugin_add_custom_Box',1 );

/* Do something with the data entered */
add_action( 'save_post','myplugin_save_postdata' );

/* Adds a Box to the main column on the Post and Page edit screens */
function myplugin_add_custom_Box() {
add_Meta_Box(
'myplugin_sectionid',
'My_custom_Box',// 可自行修改标题文字
'myplugin_inner_custom_Box',
'post'
);
}

/* Prints the Box content */
function myplugin_inner_custom_Box( $post ) {
global $wpdb;

// Use nonce for verification
wp_nonce_field( plugin_basename( __FILE__ ),'myplugin_noncename' );

// 获取固定字段keywords和description的值，用于显示之前保存的值
// 此处wp_posts新添加的字段为keywords和description，多个用半角逗号隔开
$date = $wpdb->get_row( $wpdb->prepare( "SELECT keywords,description FROM $wpdb->posts WHERE ID = %d",$post->ID) );

// Keywords 字段输入框的HTML代码
echo '<label for="keywords_new_field">Keywords</label> ';
echo '<input type="text" id="keywords_new_field" name="keywords_new_field" value="'.$date->keywords.'" size="18" />';

// description 字段输入框的HTML代码，即复制以上两行代码，并将keywords该成description
echo '<label for="description_new_field">Description</label> ';
echo '<input type="text" id="description_new_field" name="description_new_field" value="'.$date->description.'" size="18" />';
// 多个字段依此类推
}

/* 文章提交更新后，保存固定字段的值 */
function myplugin_save_postdata( $post_id ) {
// verify if this is an auto save routine.
// If it is our form has not been submitted,so we dont want to do anything
if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
return;

// verify this came from the our screen and with proper authorization,
// because save_post can be triggered at other times
if ( !wp_verify_nonce( $_POST['myplugin_noncename'],plugin_basename( __FILE__ ) ) )
return;

// 权限验证
if ( 'post' == $_POST['post_type'] ) {
if ( !current_user_can( 'edit_post',$post_id ) )
return;
}

// 获取编写文章时填写的固定字段的值，多个字段依此类推
$keywords = $_POST['keywords_new_field'];
$description = $_POST['description_new_field'];

// 更新数据库，此处wp_posts新添加的字段为keywords和description，多个根据你的情况修改
global $wpdb;
$wpdb->update( "$wpdb->posts",
// 以下一行代码，多个字段的话参照下面的写法，单引号中是字段名，右边是变量值。半角逗号隔开
array( 'keywords' => $keywords,'description' => $description ),
array( 'ID' => $post_id ),
// 添加了多少个新字段就写多少个%s，半角逗号隔开
array( '%s','%s' ),
array( '%d' )
);
}

// $post->ID是文章id，自行修改
$date = $wpdb->get_row( $wpdb->prepare( "SELECT keywords FROM $wpdb->posts WHERE ID = %d",$post->ID) );

// keywords值
$keywords = $date->keywords;

// description值
$description = $date->description;