How to write a computable verification code in php

<?php 
session_start(); 
if(!emptyempty($_POST[&#39;Login&#39;])){ 
  if(md5(strtoupper($_POST[&#39;Login&#39;])) == $_SESSION[&#39;Login&#39;]){ 
    echo &#39;Correct&#39;; 
  }else{ 
    echo &#39;Error&#39;;   
  } 
} 
?> 
 
<form method="post"> 
  <input type="text" name="Login" value="" /><img src="img.php?Action=Login&imgW=80&imgH=30" align="absmiddle"> 
  <input type="submit" value="Login" /> 
</form>

<?php 
session_start(); 
 /** 
* 随机的数字,之和验证码 
* 修改日期 2006-12-20 
*/ 
function getCode ($length = 32, $mode = 0) 
{ 
switch ($mode) 
{ 
   case &#39;1&#39;: 
     $str = &#39;123456789&#39;; 
      break; 
 case &#39;2&#39;: 
      $str = &#39;abcdefghijklmnopqrstuvwxyz&#39;; 
      break; 
 case &#39;3&#39;: 
      $str = &#39;ABCDEFGHIJKLMNOPQRSTUVWXYZ&#39;; 
      break; 
 case &#39;4&#39;: 
      $str = &#39;ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz&#39;; 
      break; 
 case &#39;5&#39;: 
      $str = &#39;ABCDEFGHIJKLMNPQRSTUVWXYZ123456789&#39;; 
      break; 
    case &#39;6&#39;: 
      $str = &#39;abcdefghijklmnopqrstuvwxyz1234567890&#39;; 
      break; 
    default: 
      $str = &#39;ABCDEFGHIJKLMNPQRSTUVWXYZabcdefghijkmnpqrstuvwxyz23456789&#39;; 
      break; 
} 
 
$result = &#39;&#39;; 
$l = strlen($str)-1; 
for($i = 0;$i < $length;$i ++) 
{ 
 $num = rand(0, $l); 
 $result .= $str[$num]; 
} 
return $result; 
} 
//建立验证图片 
function createAuthNumImg($randStr,$imgW=100,$imgH=40,$fontName) 
{ 
header ("content-type: image/png"); 
$image = imagecreate($imgW , $imgH); 
$color_white = imagecolorallocate($image , 255 , 255 , 255); 
$color_gray = imagecolorallocate($image , 228 , 228 , 228); 
$color_black = imagecolorallocate($image , 255 , 102 , 204); 
for ($i = 0 ; $i < 1000 ; $i++) 
{ 
 imagesetpixel($image , mt_rand(0 , $imgW) , mt_rand(0 , $imgH) , $color_gray); 
} 
imagerectangle($image , 0 , 0 , $imgW - 1 , $imgH - 1 , $color_gray); 
for ($i=10;$i<$imgH;$i+=10) 
 imageline($image, 0, $i, $imgW, $i, $color_gray); 
imagettftext($image,16,5,3,25,$color_black,$fontName,$randStr); 
for ($i=10;$i<$imgW;$i+=10) 
 imageline($image, $i, 0, $i, $imgH, $color_gray); 
imagepng($image); 
imagedestroy($image); 
} 
 
$a=GetCode(1,1); 
$b=GetCode(1,1); 
$c=GetCode(1,1); 
$Passport=$a."+".$b."+".$c; 
$Total=$a+$b+$c; 
$Total; 
$_SESSION[$_GET[&#39;Action&#39;]]=md5(strtoupper($Total)); 
createAuthNumImg($Passport,$_GET[&#39;imgW&#39;],$_GET[&#39;imgH&#39;],"verdana.ttf"); 
?>