How to create a 20 MB file quickly in PHP

create_20_mb_file.php

<?php
/** @link https://github.com/google/google-api-php-client/blob/master/examples/fileupload.php */
/************************************************
  We'll setup an empty 20MB file to upload.
 ************************************************/
DEFINE("TESTFILE", 'testfile.txt');
if (!file_exists(TESTFILE)) {
  $fh = fopen(TESTFILE, 'w');
  fseek($fh, 1024*1024*20);
  fwrite($fh, "!", 1);
  fclose($fh);
}