怎么样通过PHP编写登录验证码功能？

<?php
session_start();
//设置COOKIE或Session
function esetcookie($name,$str,$life=0){
//本函数将字符串 str 全部变小写字符串使验证码输入不区分大小写----在提交表单进行session比较同样需要次函数转化
 $_SESSION[$name]=strtolower($str);
}

//获取随机字符 此函数区分字符大小写 如果不区分大小写可加入函数strtolower
function domake_password($len) 
{ 
 $chars = array( 
  /*"a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", 
  "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", 
  "w", "x", "y", "z", "A", "B", "C", "D", "E", "F", "G", 
  "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", 
  "S", "T", "U", "V", "W", "X", "Y", "Z",*/ "0", "1", "2", 
  "3", "4", "5", "6", "7", "8", "9" 
 ); 
 $charsLen = count($chars) - 1; 
 shuffle($chars);// 将数组打乱
 $output = ""; 
 for ($i=0; $i<$len; $i++) 
 { 
  $output .= $chars[mt_rand(0, $charsLen)]; //获得一个数组元素
 } 
 return $output;
} 

//显示验证码
function ShowKey(){
 $key=domake_password(4);//获取随机值
 $set=esetcookie("checkkey",$key);//将随机值写入cookie或session
 //是否支持gd库
 if(function_exists("imagejpeg")) 
 {
 header ("Content-type: image/jpeg");
 $img=imagecreate(47,20);
 $blue=imagecolorallocate($img,102,102,102);
 $white=ImageColorAllocate($img,255,255,255);
 $black=ImageColorAllocate($img,71,71,71);
 imagefill($img,0,0,$blue);
 imagestring($img,5,6,3,$key,$white);
 for($i=0;$i<90;$i++) //加入干扰象素
 {
  imagesetpixel($img,rand()%70,rand()%30,$black);
 }
 imagejpeg($img);
 imagedestroy($img);
 }
 elseif (function_exists("imagepng"))
 {
 header ("Content-type: image/png");
 $img=imagecreate(47,20);
 $blue=imagecolorallocate($img,102,102,102);
 $white=ImageColorAllocate($img,255,255,255);
 $black=ImageColorAllocate($img,71,71,71);
 imagefill($img,0,0,$blue);
 imagestring($img,5,6,3,$key,$white);
 for($i=0;$i<90;$i++) //加入干扰象素
 {
  imagesetpixel($img,rand()%70,rand()%30,$black);
 }
 imagepng($img);
 imagedestroy($img);
 }
 elseif (function_exists("imagegif")) 
 {
 header("Content-type: image/gif");
 $img=imagecreate(47,20);
 $blue=imagecolorallocate($img,102,102,102);
 $white=ImageColorAllocate($img,255,255,255);
 $black=ImageColorAllocate($img,71,71,71);
 imagefill($img,0,0,$blue);
 imagestring($img,5,6,3,$key,$white);
 for($i=0;$i<90;$i++) //加入干扰象素
 {
  imagesetpixel($img,rand()%70,rand()%30,$black);
 }
 imagegif($img);
 imagedestroy($img);
 }
 elseif (function_exists("imagewbmp")) 
 {
 header ("Content-type: image/vnd.wap.wbmp");
 $img=imagecreate(47,20);
 $blue=imagecolorallocate($img,102,102,102);
 $white=ImageColorAllocate($img,255,255,255);
 $black=ImageColorAllocate($img,71,71,71);
 imagefill($img,0,0,$blue);
 imagestring($img,5,6,3,$key,$white);
 for($i=0;$i<90;$i++) //加入干扰象素
 {
  imagesetpixel($img,rand()%70,rand()%30,$black);
 }
 imagewbmp($img);
 imagedestroy($img);
 }
 else
 {
 //不支持验证码
 header("content-type:image/jpeg\r\n");
 header("Pragma:no-cache\r\n");
 header("Cache-Control:no-cache\r\n");
 header("Expires:0\r\n");
 $fp = fopen("data/vdcode.jpg","r"); 
 }
}
ShowKey();
?>