官网：http://www.geetest.com/ （此处应该有广告费）
示例项目：https://github.com/baijunyao/thinkphp-bjyadmin

一：注册获取key
注册；创建应用；获取key；

二：导入sdk
/ThinkPHP/Library/Org/Xb/Geetest.class.php
三：生成验证样式
/tpl/Home/Index/index.html

<div class="box">
    <h4>
        <a target="_blank" href="http://baijunyao.com/article/79">thinkphp整合系列之极验滑动验证码</a>
    </h4>
    <form id="geetest" action="{:U('Home/Index/geetest_submit_check')}" method="post">
        账号：<input type="text" name="username" value="admin">
        密码：<input type="text" name="password" value="123456">
        <input type="button" value="异步验证登录" onclick="check_verify()">
        <input type="submit" value="post提交登录">
        <div id="captcha"></div>
    </form>
</div>
<script src="/Public/statics/js/jquery-1.10.2.min.js"></script>
<script src="http://static.geetest.com/static/tools/gt.js"></script>
<script>
    var handler = function (captchaObj) {
        // 将验证码加到id为captcha的元素里
        captchaObj.appendTo("#captcha");
     };
    // 获取验证码
    $.get("{:U('Home/Index/geetest_show_verify')}", function(data) {
        // 使用initGeetest接口
        // 参数1：配置参数，与创建Geetest实例时接受的参数一致
        // 参数2：回调，回调的第一个参数验证码对象，之后可以使用它做appendTo之类的事件
        initGeetest({
            gt: data.gt,
            challenge: data.challenge,
            product: "float", // 产品形式
            offline: !data.success
        }, handler);
    },'json');
    // 检测验证码
    function check_verify(){
        // 组合验证需要用的数据
        var test=$('.geetest_challenge').val();
        var postData={
            geetest_challenge: $('.geetest_challenge').val(),
            geetest_validate: $('.geetest_validate').val(),
            geetest_seccode: $('.geetest_seccode').val()
        }
        // 验证是否通过
        $.post("{:U('Home/Index/geetest_ajax_check')}", postData, function(data) {
            if (data==1) {
                alert('验证成功');
            }else{
                alert('验证失败');
            }
        });
    }
</script>

四：验证函数
/Application/Common/Common/function.php

/**
 * geetest检测验证码
 */
function geetest_chcek_verify($data){
    $geetest_id=C('GEETEST_ID');
    $geetest_key=C('GEETEST_KEY');
    $geetest=new \Org\Xb\Geetest($geetest_id,$geetest_key);
    $user_id=$_SESSION['geetest']['user_id'];
    if ($_SESSION['geetest']['gtserver']==1) {
        $result=$geetest->success_validate($data['geetest_challenge'], $data['geetest_validate'], $data['geetest_seccode'], $user_id);
        if ($result) {
            return true;
        } else{
            return false;
        }
    }else{
        if ($geetest->fail_validate($data['geetest_challenge'],$data['geetest_validate'],$data['geetest_seccode'])) {
            return true;
        }else{
            return false;
        }
    }
}

多谢 香蕉你个banana 童鞋的提醒；补上验证的函数；
五：php 生成验证码 并 验证

/Application/Home/Controller/IndexController.class.php

    /**
     * geetest生成验证码
     */
    public function geetest_show_verify(){
        $geetest_id=C('GEETEST_ID');
        $geetest_key=C('GEETEST_KEY');
        $geetest=new \Org\Xb\Geetest($geetest_id,$geetest_key);
        $user_id = "test";
        $status = $geetest->pre_process($user_id);
        $_SESSION['geetest']=array(
            'gtserver'=>$status,
            'user_id'=>$user_id
            );
        echo $geetest->get_response_str();
    }
    /**
     * geetest submit 提交验证
     */
    public function geetest_submit_check(){
        $data=I('post.');
        if (geetest_chcek_verify($data)) {
            echo '验证成功';
        }else{
            echo '验证失败';
        }
    }
    /**
     * geetest ajax 验证
     */
    public function geetest_ajax_check(){
        $data=I('post.');
        echo intval(geetest_chcek_verify($data));
    }