Yaf_Session::del

<?php
class VerifyController extends Yaf_Controller_Abstract
{
    public function captchaAction()
    {
        $session = Yaf_Session::getInstance();
        $session->start();

        $expected = $session->get("captcha_code");

        // 一次性验证码：使用后立即删除
        $session->del("captcha_code");

        if ($expected === null
            || $this->getRequest()->getPost("code") !== $expected) {
            echo "invalid captcha";
        }
    }
}
?>