php验证码

<?php

session_start();
function random($len){
    $srcstr = "1a2s3d4f5g6hj8k9l0qwertyuiopzxcvbnm";
    mt_srand();
    $strs = "";
    for($i=0;$i<$len;$i++){
        $strs.= $srcstr[mt_rand(0,10)];

    }
    return $strs;
}

$str = random(4);
$width = 50;
$height = 25;
@header("Content-Type:image/png");
$im = imagecreate($width,$height);

$back = imagecolorallocate($im,0xFF,0xFF,0xFF);
$font = imagecolorallocate($im,41,163,238);
$pix = imagecolorallocate($im,187,230,247);

mt_srand();
for($i=0;$i<1000;$i++){
    imagesetpixel($im,mt_rand(0,$width),mt_rand(0,$height),$pix);
}
imagestring($im,5,7,5,$str,$font);
imagerectangle($im,0,0,$width-1,$height-1,$font);
imagepng($im);
imagedestroy($im);
$str=md5($str);
setcookie("authcode",$str,time()+3600,"/");
?>