'g-recaptcha-response' Problem when trying to verify user's response for reCaptcha

[Don G]

I am having a problem setting up the code to verify the user's response after checking the recaptcha box.

I am using this example code:
   https://github.com/google/ReCAPTCHA/blob/master/php/example-captcha.php

By itself, $_POST['g-recaptcha-response'] returns a long string of letters and numbers.  So far so good.

But when I use the following code from the example:

196    // Was there a reCAPTCHA response?

197    if ($_POST["g-recaptcha-response"]) {

198         $resp = $reCaptcha->verifyResponse(

199         $_SERVER["REMOTE_ADDR"],

200         $_POST["g-recaptcha-response"]

201         );

202   }

I get the following errors:
    Notice: Undefined variable: reCaptcha in [program name] on line 198
    Fatal error: Call to a member function verifyResponse() on a non-object in [program name] on line 198

Please tell me how to fix this.