code: public static function generate($mutantFile = null, $bootstrap = '', $replacingFile = null)
      {
          $loadHumbug = '';
          if ('phar:' === substr(__FILE__, 0, 5)) {
              $loadHumbug = '\Phar::loadPhar(\''
                  . str_replace('phar://', '', \Phar::running())
                  . '\', \'humbug.phar\');';
              $humbugBootstrap = 'phar://humbug.phar' . '/bootstrap.php';
          } else {
              $humbugBootstrap = realpath(__DIR__ . '/../../../bootstrap.php');
          }
  
          $file = sys_get_temp_dir() . '/humbug.phpunit.bootstrap.php';
  
          if (!is_null($mutantFile)) {
              $mutantFile = addslashes($mutantFile);
              $replacingFile = addslashes($replacingFile);
              $prepend = <<<PREPEND
  <?php
  {$loadHumbug}
  require_once '{$humbugBootstrap}';
  use Humbug\StreamWrapper\IncludeInterceptor;
  IncludeInterceptor::intercept('{$replacingFile}', '{$mutantFile}');
  IncludeInterceptor::enable();
  PREPEND;
              if (!empty($bootstrap)) {
                  $buffer = $prepend . "\nrequire_once '{$bootstrap}';";
              } else {
                  $buffer = $prepend;
              }
              file_put_contents($file, $buffer);
          } else {
              if (!empty($bootstrap)) {
                  $buffer = "<?php\n{$loadHumbug}\nrequire_once '{$humbugBootstrap}';\nrequire_once '{$bootstrap}';";
              } else {
                  $buffer = "<?php\n{$loadHumbug}\nrequire_once '{$humbugBootstrap}';";
              }
              file_put_contents($file, $buffer);
          }
      }
docs: Generate a new Job script to be executed under a separate PHP process
  
  @param null|string   $mutantFile
  @param array         $args
  @param string        $bootstrap
  @param null|string   $replacingFile
  @return string