<script src="http://code.jquery.com/jquery-latest.min.js"></script>
<script language="javascript">
    function ChangeCode(){
        var NewSecurity= "<img src='security.php?rnd="+Math.random()+"' alt='guvenlik' style='border: 1px solid #999999;' />";
        $("#security").html(NewSecurity);
        return false;
    }
</script>

<form action="kontrol.php" method="post">
    <table cellpadding="0" cellspacing="0" border="0">
        <tr>
        <th style="color: #b62463; padding-bottom: 20px; text-align:left;" colspan="4">Güvenlik Doğrulaması</th>
    </tr>
    <tr>
        <th>Güvenlik : </th>
        <th></th>
        <td>
        <div id="security"><img src="security.php" alt="guvenlik" style="border: 1px solid #999999;"></div>
        </td>
        <td><a href="javascript:;" onclick="ChangeCode();"><img    src="refresh.png" /></a></td>
    </tr>
    <tr>
        <th></th>
        <th></th>
        <td><input type="text" name="security"/></td>
        <td>(Resimdeki kodu giriniz.)</td>
    </tr>
    <tr>
        <th></th>
        <th></th>
        <td><input type="submit" value="Gönder"></td>
        <td></input></td>
    </tr>
    </table>
</form>
</html>

<?php
function olustur ()
{
    $sifre = substr(md5(rand(0,999999999999)),-5);
    if ($sifre)
    {
        session_start();
        $_SESSION["koruma"] = $sifre;

        $width     = 99;
        $height    = 34;
        $resim    = @imagecreate ($width,$height);
        $siyah      = ImageColorAllocate($resim, 0, 0, 0);
        $kirmizi      = ImageColorAllocate($resim, 182, 36, 99);
        $beyaz       = ImageColorAllocate($resim, 255, 255, 255);
        ImageFill($resim, 0, 0, $beyaz);

        $font = 'font/arial.ttf';

        imagettftext($resim, 20, 10, 10, 28, $siyah, $font, $sifre[0]);
        imagettftext($resim, 20, 0, 25, 28, $kirmizi, $font, $sifre[1]);
        imagettftext($resim, 20, -10, 40, 28, $siyah, $font, $sifre[2]);
        imagettftext($resim, 20, 0, 55, 28, $kirmizi, $font, $sifre[3]);
        imagettftext($resim, 20, 0, 70, 28, $siyah, $font, $sifre[4]);

        header("Content-type: image/png");
        ImagePng($resim);
        ImageDestroy($resim);
    }
}
olustur(); 
?>

<?php
session_start();
$girilen_kod    = trim(strip_tags($_POST['security']));
$guvenlik_kodu    = trim(strip_tags($_SESSION['koruma']));

if($girilen_kod != $guvenlik_kodu)
    {
        echo 'Güvenlik Kodu Yanlış';
    }
    else
    {
        echo 'Güvenlik Kodu Doğru';
    }
?>