If the function doesn't exist:

<?
    if (!function_exists("long2ip")) {
        function long2ip($long) {
            // Valid range: 0.0.0.0 -> 255.255.255.255
            if ($long < 0 || $long > 4294967295) return false;
            $ip = "";
            for ($i=3;$i>=0;$i--) {
                $ip .= (int)($long / pow(256,$i));
                $long -= (int)($long / pow(256,$i))*pow(256,$i);
                if ($i>0) $ip .= ".";
            }
            return $ip;
        }
    }
?>

For a 32bit safe long2ip, which can accept string or signed integer input, try:

  function safelong2ip($long) {
    $binStr = sprintf("%032s", decbin((float)$long));
    if (strlen($binStr) != 32) {
      throw new Exception("Invalid IPv4 subnet!");
    }

    $ipArr = [];
    for ($i = 0; $i < 4; ++$i) {
      $ipArr[] = bindec(substr($binStr, $i*8, 8));
    }

    return implode('.', $ipArr);
  }