<?phpheader("Content-type:text/html;charset='utf8'");error_reporting(E_ALL);define("CAPTCHA_LEN", 16); // 随机数长度$captchaSource = "0123456789abcdefghijklmnopqrstuvwxyz这是一个随机打印输出字符串的例子"; // 随机数字符源$captchaResult = ""; // 随机数返回值$captchaSentry = ""; // 随机数中间变量for($i=0;$i<CAPTCHA_LEN;$i++){    $n = rand(0, strlen($captchaSource));    if($n >= 36){        $n = 36 + ceil(($n - 36) / 3) * 3;        $captchaResult .= substr($captchaSource, $n, 3);    }else{        $captchaResult .= substr($captchaSource, $n, 1);    }}echo "随机打印输出字符串:".$captchaResult."<br>";