Paypal 發布了一個 Github 存儲庫，其中包含 IPN 監聽器的示例代碼。您可以查看 Github 存儲庫：https ://github.com/paypal/ipn-code-samples您可以在下面找到 IPN PHP 類：<?phpclass PaypalIPN{    /** @var bool Indicates if the sandbox endpoint is used. */    private $use_sandbox = false;    /** @var bool Indicates if the local certificates are used. */    private $use_local_certs = true;    /** Production Postback URL */    const VERIFY_URI = 'https://ipnpb.paypal.com/cgi-bin/webscr';    /** Sandbox Postback URL */    const SANDBOX_VERIFY_URI = 'https://ipnpb.sandbox.paypal.com/cgi-bin/webscr';    /** Response from PayPal indicating validation was successful */    const VALID = 'VERIFIED';    /** Response from PayPal indicating validation failed */    const INVALID = 'INVALID';    /**     * Sets the IPN verification to sandbox mode (for use when testing,     * should not be enabled in production).     * @return void     */    public function useSandbox()    {        $this->use_sandbox = true;    }    /**     * Sets curl to use php curl's built in certs (may be required in some     * environments).     * @return void     */    public function usePHPCerts()    {        $this->use_local_certs = false;    }    /**     * Determine endpoint to post the verification data to.     *     * @return string     */    public function getPaypalUri()    {        if ($this->use_sandbox) {            return self::SANDBOX_VERIFY_URI;        } else {            return self::VERIFY_URI;        }    }    /**     * Verification Function     * Sends the incoming post data back to PayPal using the cURL library.     *     * @return bool     * @throws Exception     */    public function verifyIPN()    {        if ( ! count($_POST)) {            throw new Exception("Missing POST Data");        }我正在使用貝寶訂閱： https://developer.paypal.com/docs/business/subscriptions/但我找不到可以指定 IPN Listener url 的地方。有任何想法嗎？