PHP - Run Another Php File, In Another Folder On The Linux Server

Hi, I have a form where a user selects a file to attach to the email. At the moment when you select a file it uploads from the user device.

How do i change this so that a user can attach a file from a folder on the server. For example the folder name is uploadinvoice

so when the user selects the browse button to attach a file it opens up the uploadinvoice folder on the server so the user can select the file from there ?

Thanks 

coding i have at moment

function ValidateEmail($email)

{

   $pattern = '/^([0-9a-z]([-.\w]*[0-9a-z])*@(([0-9a-z])+([-\w]*[0-9a-z])*\.)+[a-z]{2,6})$/i';

   return preg_match($pattern, $email);

}

if ($_SERVER['REQUEST_METHOD'] == 'POST' && isset($_POST['formid']) && $_POST['formid'] == 'form1')

{

   $mailto = $_POST['youremail'];

   $mailfrom = isset($_POST['myemail']) ? $_POST['myemail'] : $mailto;

   $subject = 'Message';

   $message = 'Message';

   $success_url = './test.php';

   $error_url = '';

   $eol = "\n";

   $error = '';

   $internalfields = array ("submit", "reset", "send", "filesize", "formid", "captcha_code", "recaptcha_challenge_field", "recaptcha_response_field", "g-recaptcha-response");

   $boundary = md5(uniqid(time()));

   $header  = 'From: '.$mailfrom.$eol;

   $header .= 'Reply-To: '.$mailfrom.$eol;

   $header .= 'MIME-Version: 1.0'.$eol;

   $header .= 'Content-Type: multipart/mixed; boundary="'.$boundary.'"'.$eol;

   $header .= 'X-Mailer: PHP v'.phpversion().$eol;

   try

   {

      if (!ValidateEmail($mailfrom))

      {

         $error .= "The specified email address (" . $mailfrom . ") is invalid!\n<br>";

         throw new Exception($error);

      }

      $message .= $eol;

      foreach ($_POST as $key => $value)

      {

         if (!in_array(strtolower($key), $internalfields))

         {

            if (!is_array($value))

            {

               $message .= ucwords(str_replace("_", " ", $key)) . " : " . $value . $eol;

            }

            else

            {

               $message .= ucwords(str_replace("_", " ", $key)) . " : " . implode(",", $value) . $eol;

            }

         }

      }

      $body  = 'This is a multi-part message in MIME format.'.$eol.$eol;

      $body .= '--'.$boundary.$eol;

      $body .= 'Content-Type: text/plain; charset=ISO-8859-1'.$eol;

      $body .= 'Content-Transfer-Encoding: 8bit'.$eol;

      $body .= $eol.stripslashes($message).$eol;

      if (!empty($_FILES))

      {

         foreach ($_FILES as $key => $value)

         {

             if ($_FILES[$key]['error'] == 0)

             {

                $body .= '--'.$boundary.$eol;

                $body .= 'Content-Type: '.$_FILES[$key]['type'].'; name='.$_FILES[$key]['name'].$eol;

                $body .= 'Content-Transfer-Encoding: base64'.$eol;

                $body .= 'Content-Disposition: attachment; filename='.$_FILES[$key]['name'].$eol;

                $body .= $eol.chunk_split(base64_encode(file_get_contents($_FILES[$key]['tmp_name']))).$eol;

             }

         }

      }

      $body .= '--'.$boundary.'--'.$eol;

      if ($mailto != '')

      {

         mail($mailto, $subject, $body, $header);

      }

      header('Location: '.$success_url);

   }

   catch (Exception $e)

   {

      $errorcode = file_get_contents($error_url);

      $replace = "##error##";

      $errorcode = str_replace($replace, $e->getMessage(), $errorcode);

      echo $errorcode;

   }

   exit;

}