WordPressで一括メール送信プラグインを作成

<?php
require_once( ABSPATH . WPINC . '/wp-db.php' );
class my_wpdb extends wpdb {
    var $tables = array( 'posts', 'comments', 'links', 'options', 'postmeta',
        'terms', 'term_taxonomy', 'term_relationships', 'termmeta', 'commentmeta' ,
        'mail_list');
}
if ( ! isset($wpdb) ) {
    $wpdb = new my_wpdb(DB_USER, DB_PASSWORD, DB_NAME, DB_HOST);
}
?>

<?php
/**
* @package super_mail
* @version 1.0
*/
/*
Plugin Name: 一括メール
Plugin URI: https://jisaku-hp.jp/
Description: 
Author: 廣本　隆史
Version: 1.0
Author URI: https://jisaku-hp.jp/
*/
    global $wpdb;
    $table = $wpdb->prefix.'mail_list';
    //manshonテーブルが存在していなければ作成する
    if ($wpdb->get_var("SHOW TABLES LIKE '$table'") != $table) {

        $sql = "
            CREATE TABLE IF NOT EXISTS `$table` (
id int(10) unsigned AUTO_INCREMENT NOT NULL,
type varchar(255) NOT NULL,
name varchar(255) NOT NULL,
mail_ad varchar(255) NOT NULL,
PRIMARY KEY (id)
            ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
        ";

        require_once( ABSPATH . 'wp-admin/includes/upgrade.php' );
        dbDelta( $sql );         
    }

function original_page100() {
add_menu_page('一括メール', '一括メール', 1, 'original_page100', 'csv100', '' ,201);
}
add_action('admin_menu', 'original_page100');

function csv100() {include 'mail.php';}
?>

<?php
$headers = "From:example@example.jp";
?>
<style>
textarea{display:block;}
</style>
<h2>一括メール送信</h2>
<form enctype="multipart/form-data" method="post">
<p>題名</p>
<input name="title" value="<?php echo get_option('super_title'); ?>">
<p>メール本文</p>
<textarea name="body" style="max-width:500px; width:90%; height:500px;"><?php echo get_option('super_mail'); ?></textarea>
<input type="submit" name="_update" value="更新">
<p>メール送信</p>
<select name="type" class="select">
<?php
global $wpdb;
$results = $wpdb->get_results("SELECT distinct type FROM $wpdb->mail_list");
foreach ($results as $value) {
    echo '<option value='.$value->type.'>'.$value->type.'</option>';
}
?>
</select>
<input type="submit" name="_send" value="送信">
<?php
if (isset($_POST['_update'])) {
$title = $_POST['title'];
$value = $_POST['body'];
update_option('super_title', $title);
update_option('super_mail', $value);
echo <<<EOF
<script>
    location.href='
EOF;
echo $_SERVER['REQUEST_URI'];
echo <<<EOF
';
</script>
EOF;
}
if (isset($_POST['_send'])) {
mb_language("Japanese");
mb_internal_encoding("UTF-8");
$type = $_POST['type'];
$title = get_option('super_title');
global $wpdb;
$results = $wpdb->get_results("SELECT * FROM $wpdb->mail_list WHERE type = '$type'");
foreach ($results as $value) {
$mes = $value->name."　御中\r\n\r\n";
$mes .= get_option('super_mail')."\r\n\r\n";
mb_send_mail($value->mail_ad, $title, $mes, $headers);
}
}
?>
</form>