验证码生成代码

<?
$num="";
//for($i=0;$i<4;$i++)
//{
    //$num=$num*10+rand(0,9);
//}
$num=rand(1000,9999);
Session_start();
$_SESSION["checknum"]=$num;
Header("Content-type:image/png");
$im=imagecreate(60,20);
$black=ImageColorAllocate($im,0,0,0);
$gray=ImageColorAllocate($im,200,200,200);
imagefill($im,0,0,$gray);

$style=array($black,$black,$black,$black,$black,$gray,$gray,$gray,$gray,$gray);
imagesetstyle($im,$style);
$y1=rand(0,20);
$y2=rand(0,20);
$y3=rand(0,20);
$y4=rand(0,20);
imageline($im,0,$y1,60,$y3,IMG_COLOR_STYLED);
imageline($im,0,$y2,60,$y4,IMG_COLOR_STYLED);

for($i=0;$i<80;$i++)
{
imagesetpixel($im,rand(0,60),rand(0,20),$black);    
}

$strx=rand(3,8);
for($i=0;$i<4;$i++)
{
$strpos=rand(1,6);
imagestring($im,5,$strx,$strpos,substr($num,$i,1),$black);
$strx+=rand(8,12);
}
ImagePNG($im);
ImageDestroy($im);
?>