I think you just have a typo. You have a “$” before response in your URL string so it is being interpreted as a PHP variable which at present is undefined. That should instead be an ampersand(&).

That may not be the only issue, but is the first thing I see.

I have a Contact Form in my website where I have included a reCAPTCHA. The client-side is working; however, the server-side has an issue. The server-side integration page of the Google reCAPTCHA website provides a table of the secret key, response, and remoteip. I have included them in my code but I don’t know what to include as the response since the table says:

response: Required. The user response token provided by the reCAPTCHA
client-side integration on your site.

Here is my PHP code:

    $secretKey="";
    $responseKey=$_POST['g-recaptcha-response'];
    $UserIP = $_SERVER['REMOTE_ADDR'];
    $url="...siteverify?secret=$secretKey$response=$responseKey&remoteip=$UserIP";

    $response = file_get_contents($url);
    $response = json_decode($response);

    if($response->success){
    do something
    }
    else{
    Display error message
    }

Currently, this is not working and the code is doing the else part of the if, even if the user is verified. I am not sure about the value of $response so I am guessing that that is the problem. How can I fix this issue?
Is it because of $response? If so, what value should I give to $response here:

$responseKey=$_POST['g-recaptcha'];