wordpress use selected in a dropdown list for custom post type

wp_selected-dropdown.php

<?php 
/**
 * Demonstration uses all cpt posts in a dropdown field to select 
 * a featured listings to callback in a function
 * @param $wpselected_ string References a plugin option Setting. 
 */
function wpselected_featured_listing_cb()
{
 
    $post_type = 'wpselected_post';
    $options   = get_option('wpselected_lists');
 
    // select(ed)_id will be a custom option outside of this function.
    $wpselected_select_id = $options['wpselected_featured_listing'];
 
    // Avoid ending up with no string value by setting a default value
    if( $wpselected_select_id == '' ) $wpselected_select_id = 0; 
 
    /**
     * Sets up a list of cpt posts to do foreach with
     * @param $label string Optional
     */
    $post_type_object = get_post_type_object($post_type);
    $label = $post_type_object->label;
    $posts = get_posts( array(
            'post_type'      => $post_type, 
            'post_status'     => 'publish', 
            'suppress_filters' => false, 
            'posts_per_page'    => -1
            ) 
        );
    ?>
    <label class="olmin"><?php esc_html_e( 'Listing at top home page.', 
                                           'wpselected' ); ?></label>
    <select name="wpselected_lists[wpselected_featured_listing]"
            id="ol_featured_listing">';
    <option value="0"><?php echo esc_html( $label ); ?></option>
    <?php 
    /**
     * `selected` magic in use
     */
    foreach ($posts as $post) {
        echo '<option value="' . $post->ID . '"'
             . selected( $onlist_select_id, $post->ID ) . '>'
             . $post->post_title . '</option>';   
    }
    echo '</select>';  
}