Captcha basit ornegi

gistfile1.php

<?php

// captcha.php

// Captcha image kaynagi dosyamiz resim olarak captcha kodu uretiyor
session_start();

// Captcha yanlislikla onbellekten cagrilmasin diye header ayarlarini guncelliyoruz

header("Expires: Mon, 26 Jul 1997 05:00:00 GMT");   
header("Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT");   
header("Cache-Control: no-store, no-cache, must-revalidate");   
header("Cache-Control: post-check=0, pre-check=0", false);  
header("Pragma: no-cache");   
  
function _rasgeleMetin($uzunluk=5)  
{  
    $_rand_src = array(  
        array(48,57) 
        , array(97,122) 
//      , array(65,90) 
    );  
    srand ((double) microtime() * 1000000);  
    $metin = "";  
    for($i=0;$i<$uzunluk;$i++){  
        $i1=rand(0,sizeof($_rand_src)-1);  
        $metin .= chr(rand($_rand_src[$i1][0],$_rand_src[$i1][1]));  
    }  
    return $metin;  
}  
  
$im = @imagecreatefromjpeg("captcha.jpg");   
$rand = _generateRandom(3);  
$_SESSION['captcha'] = $rand;  
ImageString($im, 5, 2, 2, $rand[0]." ".$rand[1]." ".$rand[2]." ", ImageColorAllocate ($im, 0, 0, 0));  
$rand = _generateRandom(3);  
ImageString($im, 5, 2, 2, " ".$rand[0]." ".$rand[1]." ".$rand[2], ImageColorAllocate ($im, 255, 0, 0));  
Header ('Content-type: image/jpeg');  
imagejpeg($im,NULL,100);  
ImageDestroy($im);  

?>

<?php

// giris.php
// Giris kontrolumuz 

if(!empty($_POST)) { 
    if(!isset($_SESSION['captcha']) || $_SESSION['captcha'] !== $_POST['captcha'])
        throw new RuntimeException('captcha kontrol ediniz !');
}
?>

<form role="form" method="post"> 
    <input type="text"  name="username">   
    <input type="password" name="password">   
    <img src="captcha.php" alt="captcha image"></div>
    <input type="text" name="captcha">
    <input type="submit" value="Giris">    
</form>