随机生成4位数字验证码

<?php
session_start();
srand((double)microtime()*1000000); 
while(($authnum=rand()%10000)<1000);//生成四位随机整数验证码 
$_SESSION['auth']=$authnum;

//生成验证码图片 
Header("Content-type: image/PNG");
$im = imagecreate(55,18); 
$red = ImageColorAllocate($im, 255,0,0); 
$white = ImageColorAllocate($im, 200,200,100); 
$gray = ImageColorAllocate($im, 250,250,250); 
$black = ImageColorAllocate($im, 120,120,50); 


imagefill($im,60,20,$gray); 

//将四位整数验证码绘入图片 
//位置交错
for ($i = 0; $i < strlen($authnum); $i++)
{
    $i%2 == 0?$top = -1:$top = 3;
    imagestring($im, 6, 13*$i+4, 1, substr($authnum,$i,1), $white); 
}

for($i=0;$i<100;$i++)   //加入干扰象素 
{ 
    imagesetpixel($im, rand()%70 , rand()%30 , $black); 
} 

ImagePNG($im); 
ImageDestroy($im);
?>