<?php
function custom_meta_box_markup($object)
{
    wp_nonce_field(basename(__FILE__), "meta-box-nonce");

    ?>
        <div>
            <label for="meta-box-text">Text</label>
            <input name="meta-box-text" type="text" value="<?php echo get_post_meta($object->ID, "meta-box-text", true); ?>">

            <br>

            <label for="meta-box-dropdown">Dropdown</label>
            <select name="meta-box-dropdown">
                <?php
                    $option_values = array(1, 2, 3);

                    foreach($option_values as $key => $value)
                    {
                        if($value == get_post_meta($object->ID, "meta-box-dropdown", true))
                        {
                            ?>
                                <option selected><?php echo $value; ?></option>
                            <?php
                        }
                        else
                        {
                            ?>
                                <option><?php echo $value; ?></option>
                            <?php
                        }
                    }
                ?>
            </select>

            <br>

            <label for="meta-box-checkbox">Check Box</label>
            <?php
                $checkbox_value = get_post_meta($object->ID, "meta-box-checkbox", true);

                if($checkbox_value == "")
                {
                    ?>
                        <input name="meta-box-checkbox" type="checkbox" value="true">
                    <?php
                }
                else if($checkbox_value == "true")
                {
                    ?>
                        <input name="meta-box-checkbox" type="checkbox" value="true" checked>
                    <?php
                }
            ?>
        </div>
    <?php
}

function add_custom_meta_box()
{
    add_meta_box("demo-meta-box", "Custom Meta Box", "custom_meta_box_markup", "post", "side", "high", null);
}

add_action("add_meta_boxes", "add_custom_meta_box");

function save_custom_meta_box($post_id, $post, $update)
{
    if (!isset($_POST["meta-box-nonce"]) || !wp_verify_nonce($_POST["meta-box-nonce"], basename(__FILE__)))
        return $post_id;

    if(!current_user_can("edit_post", $post_id))
        return $post_id;

    if(defined("DOING_AUTOSAVE") && DOING_AUTOSAVE)
        return $post_id;

    $slug = "post";
    if($slug != $post->post_type)
        return $post_id;

    $meta_box_text_value = "";
    $meta_box_dropdown_value = "";
    $meta_box_checkbox_value = "";

    if(isset($_POST["meta-box-text"]))
    {
        $meta_box_text_value = $_POST["meta-box-text"];
    }
    update_post_meta($post_id, "meta-box-text", $meta_box_text_value);

    if(isset($_POST["meta-box-dropdown"]))
    {
        $meta_box_dropdown_value = $_POST["meta-box-dropdown"];
    }
    update_post_meta($post_id, "meta-box-dropdown", $meta_box_dropdown_value);

    if(isset($_POST["meta-box-checkbox"]))
    {
        $meta_box_checkbox_value = $_POST["meta-box-checkbox"];
    }
    update_post_meta($post_id, "meta-box-checkbox", $meta_box_checkbox_value);
}

add_action("save_post", "save_custom_meta_box", 10, 3);
?>