framework/captcha/CaptchaAction.php
public function validate($input, $caseSensitive)
    {
        $code = $this->getVerifyCode();
        $valid = $caseSensitive ? ($input === $code) : strcasecmp($input, $code) === 0;
        $session = Yii::$app->getSession();
        $session->open();
        $name = $this->getSessionKey() . 'count';
        $session[$name] += 1;
        if ($valid || $session[$name] > $this->testLimit && $this->testLimit > 0) {
            $this->getVerifyCode(true);
        }

        return $valid;
    }
public function validate($input, $caseSensitive)
    {
        $code = $this->getVerifyCode();
        $valid = $caseSensitive ? ($input === $code) : strcasecmp($input, $code) === 0;
        $session = Yii::$app->getSession();
        $session->open();
        $name = $this->getSessionKey() . 'count';
        $session[$name] += 1;
        if ($valid || $session[$name] > $this->testLimit && $this->testLimit > 0) {
            $this->getVerifyCode(true);
        }

        return $valid;
    }
Validates the input to see if it matches the generated code.
@param string $input user input
@param bool $caseSensitive whether the comparison should be case-sensitive
@return bool whether the input is valid