[PHP] Jak najprostsza captcha OR math captcha $_POST czy parsowanie?

<?php
// getcaptcha.php
$val1 = rand(1,9);
$val2 = rand(1,9);
$val3 = rand(1,9);

// number result
$result = $val1 * $val2 + $val3;
session_start();
$_SESSION['cap'] = $result;
// text result to draw
$text = $val1." * ".$val2." + ".$val3." = ";

// making image
    $image = imagecreate(120,30);
    $bg_color = ImageColorAllocate($image, 110,0,0);
    $text_color = imagecolorallocate($image, 124, 163, 255);


$font = "/usr/share/fonts/truetype/msttcorefonts/arial.ttf";
// drawing char by char
for($x=0; $x <= strlen($text); $x++)
{
imagettftext($image, rand(12,14), rand(-15,15), 5+10*$x, 20, $text_color, $font, $text[$x]);
    }
    header("content-type: image/png");
    imagepng($image);
    imagedestroy($image);
?>

<form action="SOME_URL" method="post">
<img src="getcaptcha.php"/><input type="text" name="cap"  cols="100" rows="1" >
<input type="submit" name="send" value="Publikuj" />
</form>

session_start();
if ( isset($_SESSION['cap']) &&  ($_SESSION['cap'] == $_POST['cap']))
{
  // action
  unset($_SESSION['cap']);
}
else
{
 // wrong
}