一个完整php验证码程序(有注释)

<?php
//文件来源：新程php培训(http://www.phppx.com),转载请保留版权
session_start();

$enablegd = 1;
//判断图像处理函数是否存在
$funcs = array('imagecreatetruecolor','imagecolorallocate',
'imagefill','imagestring','imageline','imagerotate','imagedestroy',
'imagecolorallocatealpha','imageellipse','imagepng');
foreach($funcs as $func)
{
    if(!function_exists($func))
    {
        $enablegd = 0;
        break;
    }
}

ob_clean(); //清理缓冲

if($enablegd)
{
    //create captcha
    $consts = 'cdfgkmnpqrstwxyz23456';
    $vowels = 'aek23456789';
    for ($x = 0; $x < 6; $x++)
    {
        $const[$x] = substr($consts, mt_rand(0,strlen($consts)-1),1); //获取$consts中的一个随机数
        $vow[$x] = substr($vowels, mt_rand(0,strlen($vowels)-1),1); //获取$vowels中的一个随机数
    }
    $radomstring = $const[0] . $vow[0] .$const[2] . $const[1] . $vow[1] . $const[3] . $vow[3] . $const[4];
    $_SESSION['checkcode'] = $string = substr($radomstring,0,4); //显示4个字符

    $imageX = strlen($radomstring)*8;    //图像的宽
    $imageY = 20;                        //图像的高
    $im = imagecreatetruecolor($imageX,$imageY); //新建一个真彩色图像

    //creates two variables to store color
    $background = imagecolorallocate($im, rand(180, 250), rand(180, 250), rand(180, 250)); //背景色
    $foregroundArr = array(imagecolorallocate($im, rand(0, 20), rand(0, 20), rand(0, 20)),
        imagecolorallocate($im, rand(0, 20), rand(0, 10), rand(245, 255)),
        imagecolorallocate($im, rand(245, 255), rand(0, 20), rand(0, 10)),
        imagecolorallocate($im, rand(245, 255), rand(0, 20), rand(245, 255))
    );
    $foreground2 = imagecolorallocatealpha($im, rand(20, 100), rand(20, 100), rand(20, 100),80); //分配颜色并说明透明度
    $middleground = imagecolorallocate($im, rand(200, 160), rand(200, 160), rand(200, 160)); //中间背景
    $middleground2 = imagecolorallocatealpha($im, rand(180, 140), rand(180, 140), rand(180, 140),80); //中间背景2

    //与左上角的颜色相同的都会被填充
    imagefill($im, 0, 0, imagecolorallocate($im, 250, 253, 254));
    //往图像上写入文字
    imagettftext($im, 12, rand(30, -30), 5, rand(14, 16), $foregroundArr[rand(0,3)], XINCHENG_ROOT.'include/fonts/ALGER.TTF', $string[0]);
    imagettftext($im, 12, rand(50, -50), 20, rand(14, 16), $foregroundArr[rand(0,3)], XINCHENG_ROOT.'include/fonts/ARIALNI.TTF', $string[1]);
    imagettftext($im, 12, rand(50, -50), 35, rand(14, 16), $foregroundArr[rand(0,3)], XINCHENG_ROOT.'include/fonts/ALGER.TTF', $string[2]);
    imagettftext($im, 12, rand(30, -30), 50, rand(14, 16), $foregroundArr[rand(0,3)], XINCHENG_ROOT.'include/fonts/arial.ttf', $string[3]);

    //画边框
    $border = imagecolorallocate($im, 133, 153, 193);
    imagerectangle($im, 0, 0, $imageX - 1, $imageY - 1, $border);

    //画一些随机出现的点
    $pointcol = imagecolorallocate($im, rand(0,255), rand(0,255), rand(0,255));
    for ($i=0;$i<80;$i++)
    {
        imagesetpixel($im,rand(2,$imageX-2),rand(2,$imageX-2),$pointcol);
    }
    //画随机出现的线
    for ($x=0; $x<9;$x++)
    {
        if(mt_rand(0,$x)%2==0)
        {
            imageline($im, rand(0, 120), rand(0, 120), rand(0, 120), rand(0, 120), rand(0, 999999)); //画线
            imageellipse($im, rand(0, 120), rand(0, 120), rand(0, 120), rand(0, 120), $middleground2); //画椭圆
        }
        else
        {
            imageline($im, rand(0, 120), rand(0, 120), rand(0, 120), rand(0, 120), rand(0, 999999));
            imageellipse($im, rand(0, 120), rand(0, 120), rand(0, 120), rand(0, 120), $middleground);
        }
    }
    //output to browser
    header("content-type:image/png\r\n");
    imagepng($im);
    imagedestroy($im);
}
else
{
    $files = glob(XINCHENG_ROOT.'images/checkcode/*.jpg');
    if(!is_array($files)) die('请检查文件目录完整性:/images/checkcode/');

    $checkcodefile = $files[rand(0, count($files)-1)]; //随机其中一个文件
    $_SESSION['checkcode'] = substr(basename($checkcodefile), 0, 4); //获得文件名

    header("content-type:image/jpeg\r\n");
    include $checkcodefile;
}
?>