Input select option Change Content in Metabox

Question

I'm making a "Input select option" in Custom Metabox for my plugin, I'm not Expert in PHP I don;t know How to Save user "Input select option". Working perfectly plain PHP.

public function zon_featuress_boxx( $post ) {
    wp_nonce_field( 'zonpackk_testimonial', 'zonpackk_testimonial_nonce' );
        $data = get_post_meta( $post->ID, '_zonpackk_testimonial_key', true );
        $dataoption = isset($data['dataoption']) ? $data['dataoption'] : '';
        ?>

<?php
if (isset($_POST['dataoption'])) {
    $page = $_POST['dataoption'];
    $_SESSION['dataoption'] = $page;
} else {
    $page = $_SESSION['dataoption'];
}
?>

    <form name="myform" action="" method="post">
        <select name="dataoption" onchange="this.form.submit()">
            <option value="page1"<?php if ($page == "page1") {
                echo " selected";
            } ?>>Page 1
            </option>
            <option value="page2"<?php if ($page == "page2") {
                echo " selected";
            } ?>>Page 2
            </option>
        </select>
    </form>

<?php
switch ($page) {
    case 'page2':
        echo "string2";
        break;
    case 'page1':
        echo "string1";
        break;
}
?>

Answer 1

This is totally wrong. Metaboxes are shown on post/page editing screen, so if you submit your own form, the whole page will be submitted/refreshed. Look up what a metabox is here => SO Thread

You should use WP's own functions to deal with this. Look add_meta_box() and check this tutorial to know how you should think to this code. The example on the tut. page shows exactly what you need.

code from WP dev

function wporg_add_custom_box()
{
    $screens = ['post', 'wporg_cpt'];
    foreach ($screens as $screen) {
        add_meta_box(
            'wporg_box_id',           // Unique ID
            'Custom Meta Box Title',  // Box title
            'wporg_custom_box_html',  // Content callback, must be of type callable
            $screen                   // Post type
        );
    }
}
add_action('add_meta_boxes', 'wporg_add_custom_box');

function wporg_custom_box_html($post)
{
    $value = get_post_meta($post->ID, '_wporg_meta_key', true);
    ?>
    <label for="wporg_field">Description for this field</label>
    <select name="wporg_field" id="wporg_field" class="postbox">
        <option value="">Select something...</option>
        <option value="something" <?php selected($value, 'something'); ?>>Something</option>
        <option value="else" <?php selected($value, 'else'); ?>>Else</option>
    </select>
    <?php
}

To save your field you need to catch it when the post/page is saved by the user, hence when the save_post action is fired.

function wporg_save_postdata($post_id)
{
    if (array_key_exists('wporg_field', $_POST)) {
        update_post_meta(
            $post_id,
            '_wporg_meta_key',
            $_POST['wporg_field']
        );
    }
}
add_action('save_post', 'wporg_save_postdata');