In the form code to achieve more and more, but with written verification code js always feel inconvenient, so learning the code implemented under php 
code to achieve more and more in the form, but written with js verification code, always inconvenient, so learning the php code under implementation. Of course, can also be packaged as a function of time in the future is also very convenient to use, but now unpackaged. I am a rookie, do not spray big brother, big brother also hope to get advice
Now talk about it simple digital verification
Recommend you step by step in accordance with the code comments, changed little semicolons and no less.
Create a new file cap_sz.php:
<?php
session_start(); //设置session,一定要在顶部
$width = 150; //设置图片宽为300像素
$height = 40; //设置图片高为40像素
$image = imagecreatetruecolor($width, $height); //设置验证码大小的函数
$bgcolor = imagecolorallocate($image, 255, 255, 255); //验证码颜色RGB为(255,255,255)#ffffff
imagefill($image, 0, 0, $bgcolor); //区域填充
$cap_code = "";
for($i=0;$i<4;$i++){
$fontsize = 7; //设置字体大小
$fontcolor = imagecolorallocate($image, rand(0,120), rand(0,120), rand(0,120));
//数字越大，颜色越浅，这里是深颜色0-120
$fontcontent = rand(0,9);
$cap_code.=$fontcontent; //.=连续定义变量
$x = ($i*150/4)+rand(5,10);
$y = rand(5,10);
//设置坐标
imagestring($image, $fontsize, $x, $y, $fontcontent, $fontcolor);
}
$_SESSION['code'] = $cap_code; //存到session
//设置干扰元素，设置雪花点
for($i=0;$i<300;$i++){
$inputcolor = imagecolorallocate($image, rand(50,200), rand(20,200), rand(50,200));
//设置颜色，20-200颜色比数字浅，不干扰阅读
imagesetpixel($image, rand(1,149), rand(1,39), $inputcolor);
//画一个单一像素的元素
}
//增加干扰元素，设置横线(先设置线的颜色，在设置横线)
for ($i=0;$i<4;$i++) {
$linecolor = imagecolorallocate($image, rand(20,220), rand(20,220),rand(20,220));
//设置线的颜色
imageline($image, rand(1,149), rand(1,39), rand(1,299), rand(1,149), $linecolor);
}
//因为有些浏览器，访问的content-type会是文本型（乱码），所以我们需要设置成图片的格式类型
header('Content-Type:image/png');
imagepng($image); //建立png函数
imagedestroy($image); //结束图形函数，消除$image
Then a new verify index.php
<?php
header("Content-Type: text/html;charset=utf-8");
if (isset($_REQUEST['code'])){
session_start();
if ($_REQUEST['code'] == $_SESSION['code']){
echo "输入正确";
}else{
echo "输入错误，请重新输入";
}
exit();
}
?>
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> //这里不能少，不然乱码
<title>验证码测试</title>
</head>
<body>
<form>
<p>验证码:<img src="cap_sz.php" onClick="this.src='cap_sz.php?nocache='+Math.random()" style="cursor:hand" alt="点击换一张"/>点击图片可更换验证码</p>
<p>请输入图片中的内容:<input type="text" name="code" value=""/></p>
<p><input type="submit" width="20px" height=19px value="提交"></input></p>
</form>
</body>
</html>Digital test:
Entered correctly: 
Error type:
Then we realize verify the following case numbers plus English
Cap_zy.php create a new file (a copy from above)
<?php
session_start(); //设置session
$width = 150; //设置图片宽为300像素
$height = 40; //设置图片高为40像素
$image = imagecreatetruecolor($width, $height); //设置验证码大小的函数
$bgcolor = imagecolorallocate($image, 255, 255, 255); //验证码颜色RGB为(255,255,255)#ffffff
imagefill($image, 0, 0, $bgcolor); //区域填充
$cap_code = "";
for($i=0;$i<4;$i++){
$fontsize = 8; //设置字体大小
$fontcolor = imagecolorallocate($image, rand(0,120), rand(0,120), rand(0,120));
//数字越大，颜色越浅，这里是深颜色0-120
$data = 'qwertyuiopasdfghjklzxcvbnmQWERTYUIOPASDFGHJKLZXCVBNM123456789'; //添加字符串
$fontcontent = substr($data, rand(0,strlen($data)),1); //去除值，字符串截取方法
$cap_code.=$fontcontent; //.=连续定义变量
$x = ($i*150/4)+rand(5,10);
$y = rand(5,10);
//设置坐标
imagestring($image, $fontsize, $x, $y, $fontcontent, $fontcolor);
}
$_SESSION['code'] = $cap_code; //存到session
//设置干扰元素，设置雪花点
for($i=0;$i<300;$i++){
$inputcolor = imagecolorallocate($image, rand(50,200), rand(20,200), rand(50,200));
//设置颜色，20-200颜色比数字浅，不干扰阅读
imagesetpixel($image, rand(1,149), rand(1,39), $inputcolor);
//画一个单一像素的元素
}
//增加干扰元素，设置横线(先设置线的颜色，在设置横线)
for ($i=0;$i<4;$i++) {
$linecolor = imagecolorallocate($image, rand(20,220), rand(20,220),rand(20,220));
//设置线的颜色
imageline($image, rand(1,149), rand(1,39), rand(1,299), rand(1,149), $linecolor);
}
//因为有些浏览器，访问的content-type会是文本型，所以我们需要设置成图片的格式类型
header('Content-Type:image/png');
imagepng($image); //建立png函数
imagedestroy($image);
Then modify the above index.php file to verify
<?php
header("Content-Type: text/html;charset=utf-8");
if (isset($_REQUEST['code'])){
session_start();
if ($_REQUEST['code'] == $_SESSION['code']){
echo "输入正确";
}else{
echo "输入错误，请重新输入";
}
exit();
}
?>
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>验证码测试</title>
</head>
<body>
<form>
<p>验证码:<img src="cap_zy.php" onClick="this.src='cap_zy.php?nocache='+Math.random()" style="cursor:hand" alt="点击换一张"/>点击图片可更换验证码</p>
<p>请输入图片中的内容:<input type="text" name="code" value=""/></p>
<p><input type="submit" width="20px" height=19px value="提交"></input></p>
</form>
</body>
</html>English CAPTCHA:
Enter the correct: 
input errors:
This is the perfect realization!