Code:
<?php

$where_form_is="http://".$_SERVER['SERVER_NAME'].strrev(strstr(strrev($_SERVER['PHP_SELF']),"/"));

session_start();
if( ($_SESSION['security_code']==$_POST['security_code']) && (!empty($_POST['security_code'])) ) { 
$fileLine = "Ime: " . $_POST['field_1'] . " 
E-mail: " . $_POST['field_2'] . " 
Mesto: " . $_POST['field_3'] . " 
Komentar: " . $_POST['field_4'] . " 

";
$filename = 'guestbook.html';
if (is_writable($filename)) {

                if (!$handle = fopen($filename, 'a'))
                {
                     echo "Cannot open file ($filename)";
                     exit;
                }
                if (fwrite($handle, $fileLine) === FALSE) {
                    echo "Cannot write to file ($filename)";
                    exit;
                }

                fclose($handle);

            } else {
                echo "The file  is not writable";
            }

include("confirm.html");
}
else {
echo "Invalid Captcha String.";
}

?>