foo-single.php

 <?php
      /**
       * Plugin Name: Foo Single
       * Plugin URI: https://example.com
       * Description: Custom plugin.
       * Version: 1.0.0
       * Author: Nilambar Sharma
       * Author URI: https://asdf.com
       * License: GPL-2.0+
       * License URI: http://www.gnu.org/licenses/gpl-2.0.txt
       */

      add_action(
        'init',
        function () {
          $number = mt_rand( 10, 100 );
          echo $number;
        }
      );