生成随机验证吗

<?php    session_start();    header("Content-type: image/PNG");    /**     * 配置方案 */    $imageWeight           = 60;    $imageHeight           = 20;    $imageFontRGB          = array('255', '255', '255');    $imageBackgroundRGB    = array('0', '0', '0');    $imagePixelNum         = 500;    $_SESSION["imagecode"] = rand(1000,9999);       /**    * 创建图片并设置颜色 */    $im    = imagecreate($imageWeight, $imageHeight);    $black = imagecolorallocate($im, $imageFontRGB[0], $imageFontRGB[1], $imageFontRGB[2]);    $gray  = ImageColorAllocate($im, $imageBackgroundRGB[0], $imageBackgroundRGB[1], $imageBackgroundRGB[2]);    imagefill($im,0,0,$gray);    /**     * 设置两条干扰线 */    $style = array($black, $black, $black, $black, $black, $gray, $gray, $gray, $gray, $gray);    imagesetstyle($im, $style);    $y1=rand(0, $imageHeight);    $y2=rand(0, $imageHeight);    $y3=rand(0, $imageHeight);    $y4=rand(0, $imageHeight);    imageline($im, 0, $y1, $imageWeight, $y3, IMG_COLOR_STYLED);    imageline($im, 0, $y2, $imageWeight, $y4, IMG_COLOR_STYLED);    /**     * 设置干扰杂点 */    for($i=0;$i<$imagePixelNum;$i++)    {     imagesetpixel($im, rand(0,255), rand(0,255), $black);    }    //将四个数字随机显示在画布上,字符的水平间距和位置都按一定波动范围随机生成    $strx=rand($imageWeight/8, $imageWeight/4);    for($i=0;$i<4;$i++)    {     $strpos=rand(0,$imageHeight/3);     imagestring($im,5,$strx,$strpos, substr($_SESSION["imagecode"], $i, 1), $black);     $strx+=rand(8,12);    }    imagepng($im);    imagedestroy($im);?>

<?php//checkNum.phpsession_start();$num1=$_SESSION[code];function random($len){$srcstr="ABCDEFGHIJKLMNPQRSTUVWXYZ123456789";mt_srand();$strs="";for($i=0;$i< $len;$i++){$strs.=$srcstr[mt_rand(0,33)];}return strtoupper($strs);}$str=random(4); //随机生成的字符串$width = 50; //验证码图片的宽度$height = 25; //验证码图片的高度@header("Content-Type:image/png");$_SESSION["code"] = $str;//echo $str;$im=imagecreate($width,$height);//背景色$back=imagecolorallocate($im,0xFF,0xFF,0xFF);//模糊点颜色$pix=imagecolorallocate($im,187,230,247);//字体色$font=imagecolorallocate($im,41,163,238);//绘模糊作用的点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);$_SESSION["code"] = $str;?>