<div class="item">
            <label>Test</label>
            <input type="text" name="keystring" size="8">
            <img src="img/img.php?" style="cursor: pointer;" alt="" onclick="javascript:this.src+=Math.ceil(Math.random()*10);" />
        </div>

if (isset($_POST['keystring']) && md5($_POST['keystring']) != $_SESSION['code_session']) {
    return false;
}

<?php

header('Content-Type: image/png');
header('Cache-control: no-cache, no-store');

session_name('phpMyAdmin');
session_start();
$im       = imagecreatetruecolor(120, 30);
$white    = imagecolorallocate($im, 255, 255, 255);
$black    = imagecolorallocate($im, 0, 0, 0);
$font     = 'arialbd.ttf';
imagefilledrectangle($im, 0, 0, 119, 29, $white);
$code = '';
$j = 10;

for($i = 0; $i < 5; $i++) {
    $number = mt_rand(0, 9);
    $color = imagecolorallocate($im, mt_rand(0, 200), mt_rand(0, 200), mt_rand(0, 200));
    imagettftext($im, mt_rand(15, 25), mt_rand(-20, 20), $j, 25, $color, $font, $number);
    $j += mt_rand(15, 23);
    $code .= $number;
}

while($i < 500) {
    imagesetpixel($im, mt_rand(0, 120), mt_rand(0, 30), $black);
    $i++;
}

$_SESSION['code_session'] = md5($code);
imagepng($im);
imagedestroy($im);
exit();

?>

        <div class="item">
            <label><?php echo $GLOBALS['strKCaptcha']; ?></label>
            <input type="text" name="keystring" size="8">
            <img src="./kcaptcha/?<?php echo session_name()?>=<?php echo session_id()?>">
        </div>

    //kcaptcha

    if (isset($_SESSION['captcha_keystring']) && $_SESSION['captcha_keystring'] == $_POST['keystring']){
        return true;
    }else{
        return false;
    }
    unset($_SESSION['captcha_keystring']);