//首先生成4位验证码

//开启session
session_start();
//数组集合
$arr = array_merge(range(0,9),range('a','z'),range('A','Z'));
//打乱数组
shuffle($arr);
//截取4位验证码
$code = array_slice($arr,0,4);
//全部转为小写
$code = strtolower(join('',$code));
var_dump($code);
//将code存入session
$_SESSION['code'] = $code;

//创建画布
$img = imagecreate(120,30);
//画布颜色
$white = imagecolorallocate($img,255,255,255);
//自定义集中颜色
$c1 = imagecolorallocate($img,14,38,54);
$c2 = imagecolorallocate($img,63,5,16);
$c3 = imagecolorallocate($img,248,248,42);
$c4 = imagecolorallocate($img,0,0,0);
//点干扰素
for ($i = 0;$i  300;$i++){
  imagesetpixel($img,rand(0,120),rand(0,30),$c1);
}
//虚线干扰素
for($j = 0;$j  200;$j++){
 imagedashedline($img,rand(0,120),rand(0,30),rand(0,120),rand(0,30),$c2);
}
//线干扰素
for ($j = 0;$j  10;$j++){
 imageline($img,rand(0,120),rand(0,30),rand(0,120),rand(0,30),$c2);
}
//字体，这个你路径对了就OK
$font = "simhei.ttf";
//向图像写入文本
imagettftext($img,18,2,40,20,$c4,$font,$code);
//以jpg格式输出，还有以png啥的，imagepng这个自己看
imagejpeg($img);
//结束之后销毁，不销毁也行，php自带垃圾回收
imagedestroy($img);

?php
  session_start();
  print_r($_POST);
  print_r($_SESSION['code']);
  //如果提交的验证码跟session里面存的一样及认证成功
  if($_POST['n3'] == $_SESSION['code']){
    echo '注册成功';
  }else{
    echo '注册失败';
  }
?>
!doctype html>
html lang="en">
head>
  meta charset="UTF-8">
  meta name="viewport"
     content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
  meta http-equiv="X-UA-Compatible" content="ie=edge">
  title>Document/title>
/head>
body>
form action="" method="post">
  input type="text" name="n1" placeholder="cc">
  input type="text" name="n2" placeholder="s">
  input type="text" name="n3">
  !--这里点击刷新验证码 -->
  img src="xxx.php" onclick="this.src='index.php?'+Math.random()" alt="">
  input type="submit" value="submit">
/form>
/body>
/html>