Might help, but this is EE’s method to find the tmp path

public function _discover_temp_path()
 {
  $attempt = array();
        $ini_path = ini_get('upload_tmp_dir');

        if ($ini_path)
        {
            $attempt[] = realpath($ini_path);
        }

  $attempt[] = sys_get_temp_dir();
  $attempt[] = @getenv('TMP');
  $attempt[] = @getenv('TMPDIR');
  $attempt[] = @getenv('TEMP');

  $valid_temps = array_filter($attempt); // remove false's

  foreach ($valid_temps as $dir)
  {
   if (is_readable($dir) && is_writable($dir))
   {
    return $dir;
   }
  }

  return FALSE;
 }