<input type="text" name="captcha" id="captcha" placeholder="验证码" maxlength="10">
                    <img src="captcha.php"  onclick="this.src='captcha.php?' + Math.random();" width="105" height="36">

<?php
session_start();

$dataObj = json_decode($HTTP_RAW_POST_DATA);
if(strtolower($_SESSION["code"]) != strtolower($dataObj->captcha)){   
    $msg = '不正确';    
}else{
    //正确
}

<?php
session_start();
header ('Content-Type: image/png');

$image=imagecreatetruecolor(100, 30);
//背景颜色为白色
$color=imagecolorallocate($image, 255, 255, 255);
imagefill($image, 20, 20, $color);

$code='';
for($i=0;$i<4;$i++){
    $fontSize=8;
    $x=rand(5,10)+$i*100/4;
    $y=rand(5, 15);
    // $data='abcdefghijklmnpqrstuvwxyz123456789';
    $data='ABCDEFGHIJKLMNOPQRSTUVWXYZ123456789';
    $string=substr($data,rand(0, strlen($data)),1);
    $code.=$string;
    $color=imagecolorallocate($image,rand(0,120), rand(0,120), rand(0,120));
    imagestring($image, $fontSize, $x, $y, $string, $color);
}
$_SESSION['code']=$code;//存储在session里

for($i=0;$i<200;$i++){
    $pointColor=imagecolorallocate($image, rand(100, 255), rand(100, 255), rand(100, 255));
    imagesetpixel($image, rand(0, 100), rand(0, 30), $pointColor);
}
for($i=0;$i<2;$i++){
    $linePoint=imagecolorallocate($image, rand(150, 255), rand(150, 255), rand(150, 255));
    imageline($image, rand(10, 50), rand(10, 20), rand(80,90), rand(15, 25), $linePoint);
}
imagepng($image);
imagedestroy($image);