How to save them when any of them are checked

您可以使用update_post_meta(), 像这样：

add_action( \'save_post\', \'op_save_offers_meta\', 10, 2 );
function op_save_offers_meta( $post_id, $post ) {
    // Make sure that we can save the meta data.
    if ( ( empty( $post_id ) || empty( $post ) || ( \'listing\' !== get_post_type( $post ) ) )
    || ( empty( $_POST[\'post_ID\'] ) || $_POST[\'post_ID\'] != $post_id || ! isset( $_POST[\'offers_meta\'] ) )
    || ( empty( $_POST[\'offers_meta_nonce\'] ) || ! wp_verify_nonce( $_POST[\'offers_meta_nonce\'], \'save-offers-meta\' ) )
    || ( ! current_user_can( \'edit_post\', $post_id ) ) ) {
        return;
    }

    $offers_ids = wp_parse_id_list( $_POST[\'offers_meta\'] );
    if ( ! empty( $offers_ids ) ) {
        update_post_meta( $post_id, \'_offers_ids\', $offers_ids );
    } else {
        delete_post_meta( $post_id, \'_offers_ids\' );
    }
}

我还改变了你的op_render_menu_meta_box():

function op_render_menu_meta_box( $post ) { // Metabox Callback Function Called
    // Get the selected \'offers\' (array of post IDs).
    $offers_ids = (array) get_post_meta( $post->ID, \'_offers_ids\', true );

    // Displays a nonce field, without referrer field.
    wp_nonce_field( \'save-offers-meta\', \'offers_meta_nonce\', false );

    // Metabox content
    $getPostsToSelect = get_posts(
                        array(
                            \'post_type\' => \'offers\',
                            \'posts_per_page\' => 3,
                        ));
    foreach ($getPostsToSelect as $aPostsToSelect) {
    ?>
    <label>
        <input 
          type=\'checkbox\' 
          name=\'offers_meta[]\' 
          class=\'postsToSelect\'
          value=\'<?php echo $aPostsToSelect->ID ?>\'
          <?php checked( true, in_array( $aPostsToSelect->ID, $offers_ids ) ); ?>
         /> 
        <?php echo $aPostsToSelect->post_title;

?>
    </label><br/>
    <?php
}
}

and then use or echo those checked offers

您可以使用get_post_meta() 检索“报价”（即。array 发布ID的数量）&mdash；就像上面一样op_render_menu_meta_box() function, 并执行标准的循环以显示帖子详细信息。示例：

$post_id = get_the_ID();

// Get the selected \'offers\' (array of post IDs).
$offers_ids = (array) get_post_meta( $post_id, \'_offers_ids\', true );

$q = new WP_Query( [
    \'post_type\' => \'offers\',
    \'post__in\'  => $offers_ids,
] );

if ( $q->have_posts() ) {
    while ( $q->have_posts() ) {
        $q->the_post();
    ?>
        <h3><?php the_title(); ?></h3>
        <p><?php the_content(); ?></p>
    <?php
    }
}

wp_reset_query();