Usage is as follows:
$encrypted = encrypt("to encrypt string", "chitgoks");
$decrypted = decrypt($encrypted, "chitgoks");

$decrypted will return to encrypt string.

function encrypt($string, $key) {
  $result = "";
  for($i=0; $i<strlen($string); $i++) {
    $char = substr($string, $i, 1);
    $keychar = substr($key, ($i % strlen($key))-1, 1);
    $char = chr(ord($char)+ord($keychar));
    $result.=$char;
  }

  return base64_encode($result);
}

function decrypt($string, $key) {
  $result = "";
  $string = base64_decode($string);

  for($i=0; $i<strlen($string); $i++) {
    $char = substr($string, $i, 1);
    $keychar = substr($key, ($i % strlen($key))-1, 1);
    $char = chr(ord($char)-ord($keychar));
    $result.=$char;
  }

  return $result;
}