如前所述，您做了一些错误的事情，即字段名称不匹配&；未定义的变量：

function checkbox(){
    global $post;

    if ( ! is_array( $user_ids = get_post_meta( $post->ID, \'user_ids\', true ) ) )
        $user_ids = array();

    $users = get_users();    
    foreach ( $users as $user ) {
        ?>

<input id="user_field_<?php echo $user->ID ?>" type="checkbox" name="user_ids[]" value="<?php echo $user->ID ?>" <?php checked( in_array( $user->ID, $user_ids ) ) ?> /> 
<label for="user_field_<?php echo $user->ID ?>"><?php echo $user->display_name ?></label><br />
        <?php
    }

    echo \'<input type="hidden" name="user_ids[]" value="0" />\';
}

/**
 * Handler for saving our checkbox states.
 * 
 * @param   int $post_id    The current post being saved.
 */
function save_detail( $post_id ) {
    if ( isset( $_POST[\'user_ids\'] ) ) {
        $user_ids = array_map( \'absint\', ( array ) $_POST[\'user_ids\'] );
        $user_ids = array_filter( $user_ids );

        update_post_meta( $post_id, \'user_ids\', $user_ids );
    }
}

add_action( \'save_post\', \'save_detail\' );