<?php
  // Cron settings
  $cron_url="http://yoursite.com/path/to/cron.php"; // Cron URL here.
  $time_interval=20; // Time interval needed (second).
  $real_time_interval=1; // Real time interval (minute).
  
  
  set_time_limit(0);
  ignore_user_abort(1);
  
  $number_of_execution=floor($real_time_interval*60/$time_interval);
  for($i=0; $i<$number_of_execution; $i++) {
    $time=microtime(1);
    file_get_contents($cron_url);
    $time=microtime(1)-$time;
    $i<$number_of_execution and sleep($time_interval-$time);
  }
?>
Done