WPでカスタム投稿に対してカスタムフィールドを設置する【プラグインなし】

functions.php

/*カスタムフィールドの設定と有効化*/
add_action('admin_menu', 'create_custom_fields');
function create_custom_fields(){
    add_meta_box(
        'item_setting',
        '商品情報入力',
        'add_custom_fields',
        'items',
        'normal',
        'high',
    );
}

functions_custom-field.php

<form method="post" action="admin.php?page=site_settings">
    <table class="myCustomField">
        <tr>
            <th>商品コード</th>
            <td><input type="text" name="item[code]" value="<?php if(!empty($item_cf['code'])){ echo esc_html($item_cf['code']) ; } ?>" placeholder="a_001"></td>
        </tr>
        <tr>
            <th>販売状態</th>
            <td>
                <label><input type="radio" name="item[status]" value="sale" <?php if(!empty($item_cf['status']) && $item_cf['status'] == 'sale'){ echo esc_html('checked'); }elseif(empty($item_cf['status'])){ echo esc_html('checked'); } ?> checked>販売中</label>
                <label><input type="radio" name="item[status]" value="soldout" <?php if(!empty($item_cf['status']) && $item_cf['status'] == 'soldout'){ echo esc_html('checked'); } ?>>売り切れ</label>
            </td>
        </tr>
        <tr>
            <th>単価</th>
            <td><span>¥</span><input type="number" name="item[price]" value="<?php if(!empty($item_cf['price'])){ echo esc_html($item_cf['price']) ; } ?>" placeholder="999"></td>
        </tr>
        <tr>
            <th>産地</th>
            <td>
                <select name="item[area]">
                    <option value="tokyo" <?php if(!empty($item_cf['area']) && $item_cf['area'] == 'tokyo'){ echo esc_html('selected'); } ?>>東京都</option>
                    <option value="kanagawa" <?php if(!empty($item_cf['area']) && $item_cf['area'] == 'kanagawa'){ echo esc_html('selected'); } ?>>神奈川県</option>
                    <option value="saitama" <?php if(!empty($item_cf['area']) && $item_cf['area'] == 'saitama'){ echo esc_html('selected'); } ?>>埼玉県</option>
                    <option value="chiba" <?php if(!empty($item_cf['area']) && $item_cf['area'] == 'chiba'){ echo esc_html('selected'); } ?>>千葉県</option>
                    <option value="ibaraki" <?php if(!empty($item_cf['area']) && $item_cf['area'] == 'ibaraki'){ echo esc_html('selected'); } ?>>茨城県</option>
                    <option value="tochigi" <?php if(!empty($item_cf['area']) && $item_cf['area'] == 'tochigi'){ echo esc_html('selected'); } ?>>栃木県</option>
                    <option value="gunma" <?php if(!empty($item_cf['area']) && $item_cf['area'] == 'gunma'){ echo esc_html('selected'); } ?>>群馬県</option>
                </select>
            </td>
        </tr>
        <tr>
            <th>商品説明</th>
            <td><textarea name="item[explain]" rows="10"><?php if(!empty($item_cf['explain'])){ echo esc_html($item_cf['explain']); } ?></textarea></td>
        </tr>
    </table>
    </form>

functions.php

//管理画面に独自のスタイルシートを読み込む
function my_admin_style(){
    $style_path = get_stylesheet_directory_uri() . '/css/admin.css';
    wp_enqueue_style('admin_style',$style_path);
}
add_action('admin_enqueue_scripts','my_admin_style');

functions.php

/*カスタムフィールドの追加とフォームの読み込み*/
function add_custom_fields(){
global $post;
$item_cf = get_post_meta($post->ID,'item_cf', true);
?>
<?php @include_once('functions_custom-field.php'); ?>
<?php
}    

functions.php

/*カスタムフィールドの保存*/
add_action('save_post','save_custom_fields');
function save_custom_fields($post_id){
    if(isset($_POST['item'])){
        update_post_meta($post_id,'item_cf', $_POST['item']);
    }
}    

single-***.php

<?php
    $item_cf = get_post_meta($post->ID,'item_cf',true);
?>
<table class="myTbl">re    /*省略*/
    <tr>
        <th>販売価格</th>
        <td><?php if(!empty($item_cf["price"])){ echo esc_html($item_cf["price"]); } ?>円</td>
    </tr>
    /*省略*/
</table>