php验证码

<?php
session_start();
$width=50;
$height=22;
$img=imagecreatetruecolor($width,$height);
$times=4;
//$arr1=range("a","z");
$arr2=range(0,9);
//$arr3=range("A","Z");
//$arr=array_merge($arr1,$arr2,$arr3);
$arr=array_merge($arr2);
$keys=array_rand($arr,$times);
$str="";
foreach($keys as $i)
$str.=$arr[$i];
$_SESSION["randValid"]=$str;
for($i=0;$i<$times*2;$i++)
{
$color=imagecolorallocate($img,rand(0,156),rand(0,156),rand(0,156));//干扰像素颜色
imageline($img,rand(0,$width),rand(0,$height),rand(0,$width),rand(0,$height),$color);
$color=imagecolorallocate($img,rand(0,255),rand(0,255),rand(0,255));
imagesetpixel($img,rand(0,$width),rand(0,$height),$color);
}
$color=imagecolorallocate($img,255,255,255);
imagestring($img,5,5,3,$str,$color);
header("content-type:image/png");
imagepng($img);
imagedestroy($img);
?>