我想更新我的选择选项（下拉列表），但我不知道哪里出错了。这是我的密码。

<?php

function display_post_options(){

    wp_nonce_field( basename( __FILE__ ), \'post_options_nonce\' ); 

?>

    <form action=\'\' id=\'post-options-form\' method=\'post\'>
    Choose Your Post Layout:
    <select name=\'post_options_select\'>
    <option id=\'right-sidebar\' value=\'right\' name=\'right_sidebar\' <?php selected(\'post_options_select\', \'right\'); ?> >Right sidebar</option>
    <option id=\'left-sidebar\' value="left" name=\'left_sidebar\' <?php selected(\'post_options_select\', \'left\'); ?>>Left Sidebar</option>
    <option id=\'no-sidebar\' value="no" name=\'no_sidebar\' <?php selected(\'post_options_select\', \'no\'); ?>>No Sidebar</option>
    </select>
    </form>


<?php
}

function post_options(){
    add_meta_box( 
        \'post-options\',
        \'Post Options\',
        \'display_post_options\',
        \'post\',
        \'advanced\',
        \'high\',
        $callback_args
    );
}

function save_post_options($post_id){

    if (if_user_can_save($post_id, \'post_options_nonce\')) {
        update_post_meta($post_id, \'post_options_select\', $_POST[\'post_options_select\']);
    }

}

function if_user_can_save($post_id, $nonce){

    //Check if post is autosave
    $is_autosave = wp_is_post_autosave($post_id);
    //Check if it is a Revision
    $is_revision = wp_is_post_revision($post_id);
    //Is the nonce valid
    $is_valid_nonce = ( isset( $_POST[ $nonce ] ) && wp_verify_nonce( $_POST[ $nonce ], basename( __FILE__ ) ) );
    return !($is_autosave || $is_revision) && $is_valid_nonce;

}

add_action(\'add_meta_boxes\', \'post_options\');
add_action( \'save_post\', \'save_post_options\', 10, 2 );
?>

更新：（请参阅注释3）这是我将html分配给变量的地方。但它会输出（selected=\'selected\'选择您的帖子布局：[选择列表]）

function display_post_options(){
    global $post;
    wp_nonce_field( basename( __FILE__ ), \'post_options_nonce\' ); 
    $post_options_select_value =  get_post_meta($post->ID,\'post_options_select\',true);

    $html = \'Choose Your Post Layout: \';
    $html .= \'<select name="post_options_select">\';
    $html .= \'<option id="right-sidebar" value="right" name="right_sidebar"\' . selected($post_options_select_value, \'right\') . \'">Right sidebar</option>\';
    $html .= \'<option id="left-sidebar" value="left" name="left_sidebar"\' . selected($post_options_select_value, \'left\') . \'" >Left Sidebar</option>\';
    $html .= \'<option id="no-sidebar" value="no" name="no_sidebar"\' . selected($post_options_select_value, \'no\') . \'">No Sidebar</option>\';
    $html .= \'</select>\';

    echo $html;

}

function display_post_options( $post){

wp_nonce_field( basename( __FILE__ ), \'post_options_nonce\' ); 
$post_options_select_value =  get_post_meta($post->ID,\'post_options_select\',true);

?>


Choose Your Post Layout:
<select name=\'post_options_select\'>
<option id=\'right-sidebar\' value=\'right\' name=\'right_sidebar\' <?php selected($post_options_select_value, \'right\'); ?> >Right sidebar</option>
<option id=\'left-sidebar\' value="left" name=\'left_sidebar\' <?php selected($post_options_select_value, \'left\'); ?>>Left Sidebar</option>
<option id=\'no-sidebar\' value="no" name=\'no_sidebar\' <?php selected($post_options_select_value, \'no\'); ?>>No Sidebar</option>
</select>

add_action( \'save_post\', \'save_post_options\', 10, 2 );

add_action( \'save_post\', \'save_post_options\', 10 );