PHP生成验证码

<?php

check_code();

function check_code($width = 100, $height = 50, $num = 4, $type = 'jpeg')
{
    $img = imagecreate($width, $height);
    $string = '';  // 文字
    for ($i = 0; $i < $num; $i++) {
        $rand = mt_rand(0, 2);
        switch ($rand) {
            case 0:
                $ascii = mt_rand(48, 57); //0-9
                break;
            case 1:
                $ascii = mt_rand(65, 90); //A-Z
                break;
            case 2:
                $ascii = mt_rand(97, 122); //a-z
                break;
        }
        //chr()
        $string .= sprintf('%c', $ascii);

    }
    //背景颜色
    imagefilledrectangle($img, 0, 0, $width, $height, randBg($img));

    //画干扰元素

    for ($i = 0; $i < 50; $i++) {

        imagesetpixel($img, mt_rand(0, $width), mt_rand(0, $height), randPix($img));

    }
    //写字
    for ($i = 0; $i < $num; $i++) {
        $x = floor($width / $num) * $i + 2;
        $y = mt_rand(0, $height - 15);

        imagechar($img, 5, $x, $y, $string[$i], randPix($img));

    }

    //imagejpeg

    $func = 'image' . $type;

    $header = 'Content-type:image/' . $type;

    if (function_exists($func)) {
        header($header);
        $func($img);
    } else {

        echo '图片类型不支持';
    }
    imagedestroy($img);
    return $string;

}
//浅色的背景
function randBg($img)
{
    return imagecolorallocate($img, mt_rand(130, 255), mt_rand(130, 255), mt_rand(130, 255));
}
//深色的字或者点这些干扰元素
function randPix($img)
{
    return imagecolorallocate($img, mt_rand(0, 120), mt_rand(0, 120), mt_rand(0, 120));
}

?>