WordPress 發表新文章自動寄信通知會員：正確寫法與大量寄信的注意事項

// 個人資料頁加上「新文章通知」勾選
add_action( 'show_user_profile', 'ahwei_notify_field' );
add_action( 'edit_user_profile', 'ahwei_notify_field' );
function ahwei_notify_field( $user ) {
    ?>
    <h2>新文章通知</h2>
    <table class="form-table" role="presentation">
        <tr>
            <th scope="row">Email 通知</th>
            <td>
                <label>
                    <input type="checkbox" name="ahwei_notify" value="1" <?php checked( get_user_meta( $user->ID, 'ahwei_notify', true ), '1' ); ?>>
                    網站有新文章時寄信通知我
                </label>
            </td>
        </tr>
    </table>
    <?php
}

// 儲存勾選結果
add_action( 'personal_options_update', 'ahwei_save_notify_field' );
add_action( 'edit_user_profile_update', 'ahwei_save_notify_field' );
function ahwei_save_notify_field( $user_id ) {
    if ( current_user_can( 'edit_user', $user_id ) ) {
        update_user_meta( $user_id, 'ahwei_notify', isset( $_POST['ahwei_notify'] ) ? '1' : '0' );
    }
}

// 文章「第一次」發佈時,排入背景工作(不在發佈當下寄信)
add_action( 'transition_post_status', function ( $new_status, $old_status, $post ) {
    if ( 'publish' !== $new_status || 'publish' === $old_status || 'post' !== $post->post_type ) {
        return;
    }
    if ( get_post_meta( $post->ID, '_ahwei_notified', true ) ) {
        return; // 通知過了(例如先取消發佈、又重新發佈)
    }
    update_post_meta( $post->ID, '_ahwei_notified', 1 );
    wp_schedule_single_event( time() + MINUTE_IN_SECONDS, 'ahwei_send_post_notice', array( $post->ID, 0 ) );
}, 10, 3 );

// 背景工作:每批寄 20 封,寄完再排下一批
add_action( 'ahwei_send_post_notice', function ( $post_id, $offset ) {
    $post = get_post( $post_id );
    if ( ! $post || 'publish' !== $post->post_status ) {
        return;
    }

    $batch = 20;
    $users = get_users( array(
        'meta_key'   => 'ahwei_notify',
        'meta_value' => '1',
        'fields'     => array( 'ID', 'user_email' ),
        'orderby'    => 'ID',
        'order'      => 'ASC',
        'number'     => $batch,
        'offset'     => $offset,
    ) );

    $site    = wp_specialchars_decode( get_option( 'blogname' ), ENT_QUOTES );
    $title   = wp_strip_all_tags( $post->post_title );
    $subject = sprintf( '【%s】新文章：%s', $site, $title );
    $message = sprintf(
        "%s 發表了新文章：\n\n%s\n%s\n\n不想再收到通知，請到個人資料頁取消勾選：\n%s",
        $site,
        $title,
        get_permalink( $post ),
        admin_url( 'profile.php' )
    );

    foreach ( $users as $user ) {
        wp_mail( $user->user_email, $subject, $message );
    }

    // 這批寄滿了,代表可能還有下一批
    if ( count( $users ) === $batch ) {
        wp_schedule_single_event(
            time() + 5 * MINUTE_IN_SECONDS,
            'ahwei_send_post_notice',
            array( $post_id, $offset + $batch )
        );
    }
}, 10, 2 );