index.php

<?php

// Cria a imagem do capthe
$im = @imagecreatetruecolor(150, 70)
    or die("Cannot Initialize new GD image stream");
// Preenche de cinza
$bg = imagecolorallocate($im, 255, 255, 255);
imagefill($im, 0, 0, $bg);

// Coloca os chanfros aleatorios na imagem
for($i=0;$i<1000;$i++) {
    $lines = imagecolorallocate($im, rand(200, 220), rand(200, 220), rand(200, 220));
    $start_x = rand(0,150);
    $start_y = rand(0,70);
    $end_x = $start_x + rand(0,5);
    $end_y = $start_y + rand(0,5);
    imageline($im, $start_x, $start_y, $end_x, $end_y, $lines);
}

$logo = imagecreatefrompng('logo.png');
$alt = rand(-130, -30);
$larg = rand(-130, -30);
imagecopy($im, $logo, $larg, $alt, 0, 0, 150 - $larg, 70 - $alt);

// Imputa as letrinhas
$letters = array_merge( range('A', 'Z') , range(2, 9) );
unset($letters[array_search('O', $letters)]);
unset($letters[array_search('Q', $letters)]);
unset($letters[array_search('I', $letters)]);
unset($letters[array_search('5', $letters)]);
unset($letters[array_search('S', $letters)]);
shuffle($letters);
$letters = array_slice($letters, 0, 4);
$secure_text = implode('', $letters);

$i = 0;
foreach ($letters as $letter) {
    $text_color = imagecolorallocate($im, rand(0,100), rand(10,100), rand(0,100));
    // imagestring($im, 10, 20+($i*25) + rand(-5, +5), 10 + rand(10, 30),  $letter, $text_color);
    imagefttext($im, 35, rand(-10, 10), 20+($i*30) + rand(-5, +5), 35 + rand(10, 30),  $text_color, './font.ttf', $letter);
    $i++;
}

header ("Content-type: image/png");
header('Pragma: no-cache');
header('Cache-Control: private, no-cache, no-cache="Set-Cookie", proxy-revalidate');

setcookie('ahctpac', md5($secure_text), time() + 1800, '/' );

imagepng($im);
imagedestroy($im);