/* Define the custom box，適用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，使用以下一行代碼
// 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', // 可自行修改標題文字
'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' );

// 獲取固定字段keywords和description的值，用于顯示之前保存的值
// 此處wp_posts新添加的字段為keywords和description，多個用半角逗號隔開
$date = $wpdb->get_row( $wpdb->prepare( "SELECT keywords, description FROM $wpdb->posts WHERE ID = %d", $post->ID) );

// Keywords 字段輸入框的HTML代碼
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 字段輸入框的HTML代碼，即復制以上兩行代碼，并將keywords該成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" />';
// 多個字段依此類推
}

/* 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;

// 權限驗證
if ( 'post' == $_POST['post_type'] ) {
if ( !current_user_can( 'edit_post', $post_id ) )
return;
}

// 獲取編寫文章時填寫的固定字段的值，多個字段依此類推
$keywords = $_POST['keywords_new_field'];
$description = $_POST['description_new_field'];

// 更新數據庫，此處wp_posts新添加的字段為keywords和description，多個根據你的情況修改
global $wpdb;
$wpdb->update( "$wpdb->posts",
// 以下一行代碼，多個字段的話參照下面的寫法，單引號中是字段名，右邊是變量值。半角逗號隔開
array( 'keywords' => $keywords, 'description' => $description ),
array( 'ID' => $post_id ),
// 添加了多少個新字段就寫多少個%s，半角逗號隔開
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;