Captcha: Crear tu propio captcha con PHP

header("Content-type: image/png");
$string = "abcdefghijklmnopqrstuvwxyz0123456789";
for($i=0;$i<5;$i++){
    $pos = rand(0,36);
    $str .= $string{$pos};
}
$img_handle = ImageCreate (60, 22) or die ("Es imposible crear la imagen");
$back_color = ImageColorAllocate($img_handle,102,102,153);
$txt_color = ImageColorAllocate($img_handle,255,255,255);
ImageString($img_handle, 31, 5, 0, $str, $txt_color);
Imagepng($img_handle);
session_start();
$_SESSION['img_number'] = $str;