php实现可运算的验证码

<?php 
session_start(); 
if(!emptyempty($_post['login'])){ 
  if(md5(strtoupper($_post['login'])) == $_session['login']){ 
    echo 'correct'; 
  }else{ 
    echo 'error';   
  } 
} 
?> 
 
<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 '1': 
     $str = '123456789'; 
      break; 
 case '2': 
      $str = 'abcdefghijklmnopqrstuvwxyz'; 
      break; 
 case '3': 
      $str = 'abcdefghijklmnopqrstuvwxyz'; 
      break; 
 case '4': 
      $str = 'abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz'; 
      break; 
 case '5': 
      $str = 'abcdefghijklmnpqrstuvwxyz123456789'; 
      break; 
    case '6': 
      $str = 'abcdefghijklmnopqrstuvwxyz1234567890'; 
      break; 
    default: 
      $str = 'abcdefghijklmnpqrstuvwxyzabcdefghijkmnpqrstuvwxyz23456789'; 
      break; 
} 
 
$result = ''; 
$l = strlen($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['action']]=md5(strtoupper($total)); 
createauthnumimg($passport,$_get['imgw'],$_get['imgh'],"verdana.ttf"); 
?>