<"Content-type: image/gif");
     imagegif($this->img);
   } elseif (function_exists("imagejpeg")) {
     header("Content-type: image/jpeg");
     imagegif($this->img);
   } elseif (imagetypes() & IMG_PNG) {
     header("Content-type: image/png");
     imagegif($this->img);
   } else {
     die("No image support in this PHP server");
   } 
 
  }
 
  //生成验证码字符串
  private function createcode() {
   $codes = "3456789abcdefghijkmnpqrstuvwxyABCDEFGHIJKLMNPQRSTUVWXY";
 
   $code = "";
 
   for($i=0; $i < $this->num; $i++) {
    $code .=$codes{rand(0, strlen($codes)-1)}; 
   }
 
   return $code;
  }
 
  //用于自动销毁图像资源
  function __destruct() {
   imagedestroy($this->img);
  }
 
 }

<"vcode.class.php";
 //构造方法
 $vcode = new Vcode(80, 30, 4);
 //将验证码放到服务器自己的空间保存一份
 $_SESSION['code'] = $vcode->getcode();
 //将验证码图片输出
 $vcode->outimg();
"htmlcode">

<"输入成功!<br>";
 }else{
  echo "输入不对!<br>";
 }
}
"reg.php" method="post">
  username: <input type="text" name="username"> <br>
  password: <input type="password" name="password"> <br>
  code: <input type="text" onkeyup="if(this.value!=this.value.toUpperCase()) this.value=this.value.toUpperCase()" size="6" name="code"> 
    <img src="/UploadFiles/2021-04-02/code.php">

PHP经典验证码类下载：
PHP验证码类.rar