How To Use Ajax For Image Verification Aka CAPTCHA

function reset_captcha() {
document.getElementsByName("captcha").value="";
with(document.getElementById('imgCaptcha')) {
src = "";
src = 'captcha.php?' + Math.random();
}
}

function validate(form) {
if(form.fullname.value=="") {alert("Please enter your name to continue, ");form.fullname.focus();return false;}
if(form.captcha.value=="") {alert("Please enter verification code.");form.captcha.focus();return false;}
return true;
}

<?
//Start the session so we can store what the security code actually is
session_start();

//Send a generated image to the browser
create_image();
exit();

function str_rand($string_length, $string_type='numeric') {
        $string_array = array('numeric','alpha','alnum','alnum_cap','alnum_low','alpha_cap','alpha_low');
        if(array_search($string_type, $string_array) !== FALSE) {
            if($string_length < 40) {
                switch($string_type) {
                    case 'numeric':
                        $string_seed = range('1','9');
                    break;
                    case 'alpha':
                        $string_seed = array_merge(range('A','Z'), range('a','z'));
                    break;
                    case 'alpha_cap':
                        $string_seed = range('A','Z');
                    break;
                    case 'alpha_low':
                        $string_seed = range('a','z');
                    break;
                    case 'alnum':
                        $string_seed = array_merge(range('1','9'), array_merge(range('A','Z'), range('a','z')));
                    break;
                    case 'alnum_cap':
                        $string_seed = array_merge(range('1','9'), range('A','Z'));
                    break;
                    case 'alnum_low':
                        $string_seed = array_merge(range('1','9'), range('a','z'));
                    break;
                }
                $string_random = '';
                for($i=0;$i<$string_length;$i++) {
                    $string_key = array_rand($string_seed, 1);
                    $string_random .= $string_seed[$string_key];
                }
                return $string_random;
            }
            else {
                return 'Invalid string length';
            }
        }
        else {
            return 'Invalid string type';
        }
    }


function create_image()
{
$image_width = 197;
    $image_height = 40;
    $image_base = imagecreatetruecolor($image_width,$image_height);
    $image_background = imagecolorallocate($image_base, 250, 250, 250);
    imagefill($image_base, 0, 0, $image_background);
    if(isset($_GET['s'])) {
        $image_text = $_GET['s'];
    }
    else {
        $image_text = str_rand(6, 'alnum');
    }


    //Set the session to store the security code
    $_SESSION["security_code"] = $image_text;


    $image_text_array = str_split($image_text, 1);
    $image_text_color = imagecolorallocate($image_base,0,0,0);
    $image_font = 'verdana.ttf';
    $image_letter = ($image_width/8);
    header('Content-type: image/png');
    for($i=0;$i<=16;$i++) {
        $image_line_color = imagecolorallocatealpha($image_base,mt_rand('0','200'),mt_rand('0','200'),mt_rand('0','200'),mt_rand('20','115'));
        imagesetthickness($image_base,mt_rand('1','3'));
        imageline($image_base,mt_rand('0',$image_width),mt_rand('0',$image_height),mt_rand('0',$image_width),mt_rand('0',$image_height),$image_line_color);
    }
    $image_dots_spacing = 10;
    $image_dots_y = $image_height/$image_dots_spacing;
    for($i=0;$i<=$image_dots_y;$i++) {
        $image_line_color = imagecolorallocatealpha($image_base,mt_rand('0','200'),mt_rand('0','200'),mt_rand('0','200'),mt_rand('20','115'));
        imagesetthickness($image_base,2);
    }
    for($i=0;$i<=6;$i++) {
        $image_text_direction = mt_rand('1','2');
        if($image_text_direction == 1) {
            imagettftext($image_base, 20,-mt_rand('5','15'),$image_letter,30,$image_text_color,$image_font,$image_text_array[$i]);
        }
        else {
            imagettftext($image_base, 20,mt_rand('5','15'),$image_letter,30,$image_text_color,$image_font,$image_text_array[$i]);
        }
        $image_letter = $image_letter+($image_width/8);
    }
    imagepng($image_base);
    imagedestroy($image_base);
}
?>