$captcha = $_POST['captcha'];
session_start();
if (($captcha != $_SESSION['captcha']) || !$captcha) {
    // Verification failed... Do what you will.
}
unset($_SESSION['captcha']);

<span><img src='captcha.php' alt='Captcha image' id='captcha_img' /></span>

<a href='#' onclick="var i = document.getElementById('captcha_img'); var n = new Image(); var rand = (Math.round((Math.random()*1000)+1)); n.onload = function() { i.parentNode.appendChild(n); i.parentNode.removeChild(i); n.id='captcha_img'; }; n.src='captcha.php?'+rand; return false;">reload image</a>

<?php
/**
 * Generate a CAPTCHA and display the image
 *
 * @author Constantinos Neophytou
 * @link http://www.cneophytou.com/
 * @license Creative Commons Attribution-Noncommercial-Share Alike 3.0  License
 *     {@link http://creativecommons.org/licenses/by-nc-sa/3.0/}
 **/
 
/**
 * Initialize page
 **/
 
session_start();
 
header("Content-Type: image/png");
header("Cache-Control: no-cache, must-revalidate"); // HTTP/1.1
header("Expires: Mon, 26 Jul 1997 05:00:00 GMT"); // Date in the past
header("Last-Modified: ".gmdate("D, d M Y H:i:s")." GMT");
 
create_image();
exit;
 
/**
 * Generate a random string, and create a CAPTCHA image out of it
 */
function create_image()
{
    // generate pronouncable pass
    $pass = genPassword(5, 6);
    $font = './captcha.ttf';
    $maxsize = 50;
    $sizeVar = 25;
    $rotate = 20;
    $bgcol = 50; // + 50
    $bgtextcol = 80; // + 50
    $textcol = 205; // + 50
 
    // remember the pass
    $_SESSION["captcha"] = $pass;
 
    // calculate dimentions required for pass
    $box = @imageTTFBbox($maxsize,0,$font,$pass);
    $minwidth = abs($box[4] - $box[0]);
    $minheight = abs($box[5] - $box[1]);
 
    // allow spacing for rotating letters
    $width = $minwidth + 100;
    $height = $minheight + rand(5,15); // give some air for the letters to breathe
    // create initial image
    $image = ImageCreatetruecolor($width, $height);  
 
    if (function_exists('imageantialias')) {
        imageantialias($image, true);
    }
    // define background color - never the same, close to black
    $clr_black = ImageColorAllocate($image, rand($bgcol, $bgcol+30),
                                    rand($bgcol, $bgcol+30), rand($bgcol, $bgcol+30));
    imagefill($image, 0, 0, $clr_black);
 
    // calculate starting positions for letters
    $x = rand(10, 25);//($width / 2) - ($minwidth / 2);
    $xinit = $x;
    $y = ($minheight-abs($box[1])) + (($height - $minheight) / 2);
 
    // fill the background with big letters, colored a bit lightly, to vary the bg.
    $bgx = $x / 2;
    $size = rand($maxsize - 10, $maxsize);
    for($i = 0; $i < strlen($pass); $i++) {
        // modify color a bit
        $clr_white = ImageColorAllocate($image, rand($bgtextcol, $bgtextcol+50),
                    rand($bgtextcol, $bgtextcol+50), rand($bgtextcol, $bgtextcol+50));
        $angle = rand(0-$rotate, $rotate);
        $letter = substr($pass, $i, 1);
        imagettftext($image, $size*2, $angle, $bgx, $y, $clr_white, $font, $letter);
        list($x1, $a, $a, $a, $x2) = @imageTTFBbox($size,$angle,$font,$letter);
        $bgx += abs($x2 - $x1);
    }
 
    // for each letter, decide a color, decide a rotation, put it on the image,
    //     and figure out width to place next letter correctly
    for($i = 0; $i < strlen($pass); $i++) {
        // modify color a bit
        $clr_white = ImageColorAllocate($image, rand($textcol, $textcol+50), 
                            rand($textcol, $textcol+50), rand($textcol, $textcol+50));
 
        $angle = rand(0-$rotate, $rotate);
        $letter = substr($pass, $i, 1);
        $size = rand($maxsize - $sizeVar, $maxsize);
        $tempbox = @imageTTFBbox($size,$angle,$font,$letter);
 
        $y = (abs($tempbox[5] - $tempbox[1])) +
                                        (($height - abs($tempbox[5] - $tempbox[1])) / 2);
 
        imagettftext($image, $size, $angle, $x, $y, $clr_white, $font, $letter);
        $x += abs($tempbox[4]-$tempbox[0]);
    }
    // figure out final width (same space at the end as there was at the beginning)
    $width = $xinit + $x;
 
    // throw in some lines
    $clr_white = ImageColorAllocate($image, rand(160, 200), rand(160, 200),
                                             rand(160, 200));
    imagelinethick($image, rand(0, 10), rand(0, $height / 2), rand($width - 10, $width),
                           rand($height / 2, $height), $clr_white, rand(1, 2));
    $clr_white = ImageColorAllocate($image, rand(160, 200), rand(160, 200),
                                             rand(160, 200));
    imagelinethick($image, rand(($width / 2) - 10, $width / 2),
                            rand($height / 2, $height), rand(($width / 2)+ 10, $width),
                           rand(0, ($height / 2)), $clr_white, rand(1, 2));
 
    // generate final image by cropping initial image to the proper width,
    //     which we didn't know till now.
    $finalimage = ImageCreatetruecolor($width, $height);
    if (function_exists('imageantialias')) {
        imageantialias($finalimage, true);
    }
    imagecopy($finalimage, $image, 0, 0, 0, 0, $width, $height);
    // clear some memory
    imagedestroy($image);
 
    // dump image
    imagepng($finalimage);
 
    // clear some more memory
    imagedestroy($finalimage);
}
 
/**
 * Draw lines through an image
 * @param resource
 * @param int
 * @param int
 * @param int
 * @param int
 * @param int - the color to use for the line
 * @param int - line thickness
 * @return bool - true on success
 */
function imagelinethick($image, $x1, $y1, $x2, $y2, $color, $thick = 1)  {
   if ($thick == 1) {
       return imageline($image, $x1, $y1, $x2, $y2, $color);
   }
   $t = $thick / 2 - 0.5;
   if ($x1 == $x2 || $y1 == $y2) {
       return imagefilledrectangle($image, round(min($x1, $x2) - $t),
                                           round(min($y1, $y2) - $t),
                                           round(max($x1, $x2) + $t),
                                           round(max($y1, $y2) + $t),
                                   $color);
   }
   $k = ($y2 - $y1) / ($x2 - $x1); //y = kx + q
   $a = $t / sqrt(1 + pow($k, 2));
   $points = array(
       round($x1 - (1+$k)*$a), round($y1 + (1-$k)*$a),
       round($x1 - (1-$k)*$a), round($y1 - (1+$k)*$a),
       round($x2 + (1+$k)*$a), round($y2 - (1-$k)*$a),
       round($x2 + (1-$k)*$a), round($y2 + (1+$k)*$a),
   );    
   imagefilledpolygon($image, $points, 4, $color);
   return imagepolygon($image, $points, 4, $color);
}
 
/**
 * Generate a random, pronouncable password
 * Modified to exclude letters which don't show up well in the CAPTCHA
 * @link http://www.zend.com/code/codex.php?id=215&single=1
 * @author Rival7 {@link http://www.zend.com/code/search_code_author.php?author=Rival7}
 * @author Constantinos Neophytou
 */
function genPassword($minlen, $maxlen) {
    srand((double)microtime()*1000000);
 
    $vowels = array('a', 'e', 'i', 'o', 'u');
    $cons = array('b', 'c', 'd', 'g', 'h', 'j', 'k', 'm', 'n', 'p', 'r', 's', 't', 'v',
     'w', 'tr', 'cr', 'br', 'fr', 'th', 'dr', 'ch', 'ph', 'wr', 'st', 'sp', 'sw', 'pr');
    // removed 'l', 'sl', 'cl'
 
    $num_vowels = count($vowels);
    $num_cons = count($cons);
 
    $length = rand($minlen, $maxlen);
 
    $start = rand(0, 1);
    if ($start) {
        $first = $cons;
        $num_first = $num_cons;
        $second = $vowels;
        $num_second = $num_vowels;
    } else {
        $first = $vowels;
        $num_first = $num_vowels;
        $second = $cons;
        $num_second = $num_cons;
    }
 
    for($i = 0; $i < $length; $i++) {
        $add = $first[rand(0, $num_first - 1)] . $second[rand(0, $num_second - 1)];
        $password .= $add;
        $i += (strlen($add) - 1);
    }
 
    //$password = substr($password, 0, $length);
    return $password;
}
 
?>