在哪里，我把reCaptcha php放在我现有的表格上？

Question

I'm trying to implement reCaptcha into my existing contact form, and have hit a snag with where exactly (notice I used the word exactly) to place the server side PHP code within my page.

I've added the required PHP within the form with correct public key (and added the private key to the server side PHP).

I have the validation PHP for the form at the top of the same page as the form is on.

Existing validation code as follows:

<?php
 // Set email variables
$email_to = 'myemailishere';
$email_subject = 'MY Enquiry TITLE IS HERE';

// Set required fields
$required_fields = array('fullname','email','comment');

// set error messages
$error_messages = array(
    'fullname' => 'Please enter your Name.',
    'email' => 'Please enter a valid Email.',
    'comment' => 'Please enter a Message.'
);


// Set form status
$form_complete = FALSE;

// configure validation array
$validation = array();

// check form submittal
if(!empty($_POST)) {
    // Sanitise POST array
    foreach($_POST as $key => $value) $_POST[$key] = remove_email_injection(trim($value));

    // Loop into required fields and make sure they match our needs
    foreach($required_fields as $field) {       
        // the field has been submitted?
        if(!array_key_exists($field, $_POST)) array_push($validation, $field);

        // check there is information in the field?
        if($_POST[$field] == '') array_push($validation, $field);


        // validate the email address supplied
        if($field == 'email') if(!validate_email_address($_POST[$field])) array_push($validation, $field);
    }

    // basic validation result
    if(count($validation) == 0) {
        // Prepare our content string
        $email_content = 'New Website Comment: ' . "

";

        // simple email content
        foreach($_POST as $key => $value) {
            if($key != 'submit') $email_content .= $key . ': ' . $value . "
";
        }

        // if validation passed ok then send the email
        mail($email_to, $email_subject, $email_content);

        // Update form switch
        $form_complete = TRUE;
    }
}

function validate_email_address($email = FALSE) {
    return (preg_match('/^[^@\s]+@([-a-z0-9]+\.)+[a-z]{2,}$/i', $email))? TRUE : FALSE;
}

function remove_email_injection($field = FALSE) {
   return (str_ireplace(array("", "
", "%0a", "%0d", "Content-Type:", "bcc:","to:","cc:"), '', $field));
}

?>

And my Form code is this:

<div id="mainform">
<?php if($form_complete === FALSE): ?>
<form autocomplete="off" action="index.php#contact" method="post" id="comments_form">
<div class="row">
<div class="label">Your full name</div><!---end label--->
<div class="input">
<input type="text" id="fullname" class="detail" name="fullname" value="<?php echo isset($_POST['fullname'])? $_POST['fullname'] : ''; ?>" /><?php if(in_array('fullname', $validation)): ?><span class="error"><?php echo $error_messages['fullname']; ?></span><?php endif; ?>
</div><!---end input--->
</div><!---end row--->

<div class="row">
<div class="label">Your email address</div><!---end label--->
<div class="input">
<input type="text" id="email" class="detail" name="email" value="<?php echo isset($_POST['email'])? $_POST['email'] : ''; ?>" /><?php if(in_array('email', $validation)): ?><span class="error"><?php echo $error_messages['email']; ?></span><?php endif; ?>
</div><!---end input--->
</div><!---end row--->

<div class="row">
<div class="label">Your number? (if you'd like a call)</div><!---end label--->
<div class="input">
<input type="text" id="telephone" class="detail" name="telephone" value="<?php echo isset($_POST['telephone'])? $_POST['telephone'] : ''; ?>" />
</div><!---end input--->
</div><!---end row--->

<div class="row">
<div class="label">Your message</div><!---end label--->
<div class="input">
<textarea id="comment" name="comment" class="mess"><?php echo isset($_POST['comment'])? $_POST['comment'] : ''; ?></textarea><?php if(in_array('comment', $validation)): ?><span class="error"><?php echo $error_messages['comment']; ?></span><?php endif; ?>
</div><!---end input--->
</div><!---end row--->

<div class="row">
<div class="label">Prove you're Human</div><!---end label--->
<?php
require_once('recaptchalib.php');
$publickey = "your_public_key"; // public key omitted for purpose of stackeroverflow
echo recaptcha_get_html($publickey);
?>
</div><!---end row--->

<div class="submit">
<input type="submit" id="submit" name="submit" value="SEND MESSAGE" />
</div><!---end submit--->
</form>

<?php else: ?>
<p>Thank you, we've received your message.</p>
<?php endif; ?>

</div><!---end mainform--->

So...where do I stick this code (as in integrate the code into my existing validation php)?????:

<?php
  require_once('recaptchalib.php');
  $privatekey = "your_private_key";
  $resp = recaptcha_check_answer ($privatekey,
                                $_SERVER["REMOTE_ADDR"],
                                $_POST["recaptcha_challenge_field"],
                                $_POST["recaptcha_response_field"]);

  if (!$resp->is_valid) {
    // What happens when the CAPTCHA was entered incorrectly
    die ("The reCAPTCHA wasn't entered correctly. Go back and try it again." .
         "(reCAPTCHA said: " . $resp->error . ")");
  } else {
    // Your code here to handle a successful verification
  }
  ?>

Hope that makes sense and someone can help?