Aprenda a fazer uma captcha (imagem de segurança)

session_start();

function captcha($width='120',$height='40',$ch='5') {

$pos = '23456789bcdfghjkmnpqrstvwxyz'; 

$i = 0;
$code = "";
while ($i < $ch) { 
  $code .= substr($pos, mt_rand(0, strlen($pos)-1), 1);
  $i++;
}

$font_size = $height * 0.6;
$image = imagecreate($width, $height);
$bg_color = imagecolorallocate($image,240,247,255);
$text_color = imagecolorallocate($image, 0, 0, 150);<p>$n_color = imagecolorallocate($image, 120, 160, 180);<br> </p>

for($i=0; $i<($width*$height)/3; $i++){
    imagefilledellipse($image, 
    mt_rand(0,$width), 
    mt_rand(0,$height), 1, 1, $n_color);
}
for( $i=0; $i<($width*$height)/160; $i++ ) {
   imageline($image,mt_rand(0,$width),mt_rand(0,$height),
   mt_rand(0,$width),mt_rand(0,$height), $n_color);
}

$font = array("1.ttf","2.ttf","3.ttf",
              "4.ttf","5.ttf","6.ttf");
$rand = array($characters);
$rand2 = array($characters);

$ii = 0;
$x = 0;
$y = $height;
while($ii < $ch){
  $ver = substr($code,$ii,1);
  $f = mt_rand(0,count($font)-1);
  $rand[$ii] = $f;
  $rand2[$ii] = $x;
  $textbox = imagettfbbox($font_size,0,$font[$f], $ver);
  $y = ($y>($height - $textbox[5])/2) ? 
  ($height - $textbox[5])/2 : $y;
  $x += $textbox[4] + 4;
  $ii++;
}

if($x - 4 > $width){
   captcha($width,$height,$characters);
   exit;
}

$x = ($width - $x)/2;
$i = 0;
while($i < $characters){
  imagettftext($image, $font_size, 0, 
  ($rand2[$i] + $x), 30, 
  $text_color, $font[$rand[$i]], substr($code,$i,1));
  $i++;
}

header('Content-Type: image/png');
imagepng($image);
imagedestroy($image);

$_SESSION["seguranca"] = $code;

}
$width = isset($_GET['width']) ? $_GET['width'] : '120';
$height = isset($_GET['height']) ? $_GET['height'] : '40';
$ch = isset($_GET['ch']) ? $_GET['ch'] : '5';
captcha($width,$height,$ch)

<img src="captcha.php?width=largura&height=altura&ch=
quantidade_de_caracteres" alt="" />

session_start();

if($_SESSION["seguranca"] != $POST["campo"]){
exit("Caracteres incorretos!");
}