Add MetaBox Wordpress

<?php
// Add FieldName MetaBox
abstract class FieldName_Meta_Box {


    /**
     * Set up and add the meta box.
     */
    public static function add() {
        $screens = [ 'posttypename' ];
        foreach ( $screens as $screen ) {
            add_meta_box(
                'fieldname_id',          // Unique ID
                'Field Name', // Box title
                [ self::class, 'html' ],   // Content callback, must be of type callable
                $screen                  // Post type
            );
        }
    }


    /**
     * Save the meta box selections.
     *
     * @param int $post_id  The post ID.
     */
    public static function save( int $post_id ) {
        if ( array_key_exists( 'fieldname_field', $_POST ) ) {
            update_post_meta(
                $post_id,
                'fieldname_meta_key',
                $_POST['fieldname_field']
            );
        }
    }


    /**
     * Display the meta box HTML to the user.
     *
     * @param \WP_Post $post   Post object.
     */
    public static function html( $post ) {
        $value = get_post_meta( $post->ID, 'fieldname_meta_key', true );
        ?>
        <label for="fieldname_field">Description for this field</label>
        <select name="fieldname_field" id="fieldname_field" class="postbox">
            <option value="">Select Location</option>
            <option value="tunis" <?php selected( $value, 'something' ); ?>>Tunis</option>
            <option value="sousse" <?php selected( $value, 'else' ); ?>>Sousse</option>
        </select>
        <?php
    }
}

add_action( 'add_meta_boxes', [ 'FieldName_Meta_Box', 'add' ] );
add_action( 'save_post', [ 'FieldName_Meta_Box', 'save' ] );