Assign membership to existing users

/**
 * A drop-in code snippet to set all users on the site with
 * a specified membership product and assign expiration based
 * on imported meta field.
 *
 * Prerequisites:
 * * (Optional) IF an existing expiration date is to be set, assign 
 *   that to the existing users via an import, using a custom meta 
 *   field for the current expiration. Set the $imported_meta to the
 *   meta key used for this field.
 *   @see: https://rocketgeek.com/blog/importing-users-using-a-plugin/
 *
 * To Use:
 * 1. Save the code snippet to your theme's functions.php
 * 2. Go to Tools > Assign User Membership.
 * 3. Follow prompts on screen.
 * 4. Remove the code snippet when completed.
 */
add_action( 'init', 'assign_membership_to_all_users_init' );
function assign_membership_to_all_users_init() {
    global $wpmem;
    $wpmem->assign_membership = new My_Assign_Membership_Class();
}
class My_Assign_Membership_Class {
 
    public function __construct() {
        add_action( 'admin_menu', array( $this, 'admin_menu' ) );
    }

    public function admin_menu() {
        $hook = add_management_page( 'Assign User Membership', 'Assign User Membership', 'edit_users', 'assign-all-users-membership', array( $this, 'admin_page' ) );
        add_action( "load-$hook", array( $this, 'admin_page_load' ) );
    }

    public function admin_page_load() {
        global $activate_all_complete;
        $activate_all_complete = false;
        if ( isset( $_GET['page'] ) && 'assign-all-users-membership' == $_GET['page'] && isset( $_POST['update-all-confirm'] ) && 1 == $_POST['update-all-confirm'] ) {
            $users = get_users( array( 'fields'=>'ID' ) );
            $membership = sanitize_text_field( $_POST['membership_to_assign'] );
            $expiration = ( isset( $_POST['import_meta_key'] ) ) ? sanitize_text_field( $_POST['import_meta_key'] ) : false;
            foreach ( $users as $user_id ) {
                $initial_expiration = ( $expiration ) ? get_user_meta( $user_id, $expiration, true ) : false;
                wpmem_set_user_product( $membership, $user_id, $initial_expiration );
            }
            $activate_all_complete = true;
        }
    }

    public function admin_page() {
        global $wpdb, $wpmem, $activate_all_complete;
        echo "<h2>Assign User Membership</h2>";
        if ( $activate_all_complete ) {
            echo '<p>All users were updated.<br />';
            echo 'You may now remove this code snippet if desired.</p>';
        } else {
            // Handle form post.
            $form_post = ( function_exists( 'wpmem_admin_form_post_url' ) ) ? wpmem_admin_form_post_url() : '';
            // Create a form field with all products in a dropdown.
            $product_selector = wpmem_form_field( array(
                'name' => 'membership_to_assign',
                'type' => 'membership',
                'required' => 1,
            ) );
            // Create a form field with all possible user meta keys in a dropdown.
            $values = $wpdb->get_results( "SELECT DISTINCT meta_key FROM " . $wpdb->usermeta );
            $exp_meta_values[] = 'Select expiration field meta key (if any)|';
            foreach ( $values as $value ) {
                $exp_meta_values[] = $value->meta_key . "|" . $value->meta_key;
            }
            $expiration_meta = wpmem_form_field( array(
                'name'  => 'import_meta_key',
                'type'  => 'select',
                'value' => $exp_meta_values,
            ) );
            echo '<form name="assign-all-users-membership" id="assign-all-users-membership" method="post" action="' . $form_post . '">';
            echo "<p>Membership to assign for all users: " . $product_selector . "</p>";
            echo '<p>For each user, if an expiration date was imported, select the meta key containing the<br />';
            echo 'expiration date (or leave blank to assign the default expiration) <br />';
            echo $expiration_meta . "</p>";
            echo '<p><input type="checkbox" name="update-all-confirm" value="1" required /><label for="update-all-confirm">Update all users? (If you do not confirm, no users will be updated.)</label></p>';
            echo '<p><input type="submit" name="submit" value="Submit" /></p>';
            echo '</form>';
        }
    }
}
// End of My_Assign_Membership_Class