Captcha with arithmetic's questions

session_start();
header("Content-type: image/png");
$width = 120;
$height = 30;
$im = @imagecreate($width, $height)
    or die("Cannot Initialize new GD image stream");
imagecolorallocate($im, 247, 250, 233);
$noise_color = imagecolorallocate($im, 253, 175, 90);
	for( $i=0; $i<($width*$height)/3; $i++ ) {
         imagefilledellipse($im, mt_rand(0,$width), mt_rand(0,$height), 1, 1, $noise_color);
      }
      /* generate random lines in background */
      for( $i=0; $i<($width*$height)/120; $i++ ) {
         imageline($im, mt_rand(0,$width), mt_rand(0,$height), mt_rand(0,$width), mt_rand(0,$height), $noise_color);
      }
$text_color = imagecolorallocate($im, 175, 102, 43);
 
 
$action = array('+', '*', '-');
shuffle($action);
if($action[0] == '+') {
	$a = rand(1, 9);
	$b = rand(1, 9);
	$_SESSION['code'] = $a + $b;
}
elseif($action[0] == '*') {
	$a = rand(1, 9);
	$b = rand(1, 9);
	$_SESSION['code'] = $a * $b;
}
elseif($action[0] == '-') {
	$b = rand(1, 9);
	$c = $b+1;
	$a = rand($c, 9);
 
	$_SESSION['code'] = $a - $b;
}
 
$text = $a.$action[0].$b;
 
for($j = 0; $j < strlen($text); $j++) {
	$angle = rand(-20, 20);
	imagettftext($im, 18, $angle, 29+($j*23), (18+(($height-18)/2)), $text_color, './WC_Wunderbach_Rough.ttf', $text[$j] );
}
imagepng($im);
imagedestroy($im);
 
//usage : save this snippet as captcha.php file and use it that
//point it as image source 
//<img src="captcha.php" />
//to check is it passed correctly, do following
//if(isset($_SESSION['code']) && ($_POST['captcha'] == $_SESSION['code'])) 
//its needed to check is $_SESSION['code'] is set, because if somebody block
//the image loading, the check ($_POST['captcha'] == $_SESSION['code']) will 
//return false positive if the form is sent with empty captcha field