Validating simple RSVP form through PHP

I am trying to validate my RSVP form using only PHP. The user should receive an error message when the form is incomplete. I am trying to avoid the use of jQuery.

I am using this tutorial: http://ift.tt/18hRCof

The form is functioning fine but I haven't been able to get the error messages to display at all. I am using Wordpress and I want the form to appear at the footer of every page; not sure if this complicates matters. Here is my code:

<?php

$response = ""; 

    //function to generate response
    function my_contact_form_generate_response($type, $message){

    global $response;

    if($type == "success") $response = "<div class='success'>{$message}</div>";
    else $response = "<div class='error'>{$message}</div>";

    }


//response messages
$missing_content = "Please supply all information.";
$email_invalid   = "Email Address Invalid.";
$message_unsent  = "Message was not sent. Try Again.";
$message_sent    = "Thanks! Your message has been sent.";

//variables defined for messages 
$email = $_POST["rsvp_email"]; 
$name = $_POST["rsvp_name"];
$attend = $_POST["rsvp_attend"];
$number = $_POST["rsvp_number"];

//variables defined for message to admin
$to = get_option('admin_email'); //sending to wordpress admin email
$subject = "Just Kidding You Foo"; 
$headers = "From: $email\n";
$message = "$name $attend.\n RSVPs $number of people";

//conditional statements used for form validation

      //validate email
      if(!filter_var($email, FILTER_VALIDATE_EMAIL))
        my_contact_form_generate_response("error", $email_invalid);
      else //email is valid
      {
        //validate presence of name and message
        if(empty($name) || empty($attend) || empty($number)){
          my_contact_form_generate_response("error", $missing_content);
        }
        else //ready to go!
        {
          $sent = wp_mail($to,$subject,$message,$headers);
          if($sent) my_contact_form_generate_response("success", $message_sent); //message sent!
          else my_contact_form_generate_response("error", $message_unsent); //message wasn't sent
        }
      }

?>


<div id="page-rsvp">        

    <h1>RSVP</h1>

    <div id="respond">

    <?php echo $response; ?>

    <form action="<?php the_permalink(); ?>" method="post">               

        <!--Name here-->
        <div class="rsvp-full"><label for="rsvp_name"><input type="text" name="rsvp_name" value="Your name"></label></div>
        <div class="rsvp-full"><label for="rsvp_email"><input type="text" name="rsvp_email" value="Your email"></label></div>

        <!--status of attendance-->
        <div class="rsvp-full">
        <div class="rsvp-element"><input id="radio-button" type="radio" name="rsvp_attend" value="accepts">Accepts</div>
        <div class="rsvp-element"><input id="radio-button" type="radio" name="rsvp_attend" value="declines">Declines</div>
        </div>    

        <!--number of guests attending-->                    
        <div class="rsvp-full"><input type="number" name="rsvp_number" min="1" max="5">Total number of guests attending</div>    

         <div id="submit-button" class="rsvp-full"><input id="submit-button" type="submit"></div>

    </form> 

    </div>  


</div>

TIA!!!