title: PHP Forms and Ajax calls, reports undefined index error after form submit
body: <p>This is my second attempt in trying to make a working contact section on my website. After reading up on debugging php I still don't know what to do next. After hitting submit I get an "undefined index" error -- and it cites multiple lines as the problem.</p>
  
  <pre><code> &lt;?php
  
  // Only process POST reqeusts.
  if ($_SERVER["REQUEST_METHOD"] == "POST") {
      // Get the form fields and remove whitespace.
      $name = strip_tags(trim($_POST["name"]));
              $name = str_replace(array("\r","\n"),array(" "," "),$name);
      $email = filter_var(trim($_POST["email"]), FILTER_SANITIZE_EMAIL);
      $message = trim($_POST["message"]);
  
      // Check that data was sent to the mailer.
      if ( empty($name) OR empty($message) OR !filter_var($email, FILTER_VALIDATE_EMAIL)) {
          // Set a 400 (bad request) response code and exit.
          http_response_code(400);
          echo "Oops! There was a problem with your submission. Please complete the form and try again.";
          exit;
      }
  
      // Set the recipient email address.
      $recipient = "someone@example.com";
  
      // Set the email subject.
      $subject = "New contact from $name";
  
      // Build the email content.
      $email_content = "Name: $name\n";
      $email_content .= "Email: $email\n\n";
      $email_content .= "Message:\n$message\n";
  
      // Build the email headers.
      $email_headers = "From: $name &lt;$email&gt;";
  
      // Send the email.
      if (mail($recipient, $subject, $email_content, $email_headers)) {
          // Set a 200 (okay) response code.
          http_response_code(200);
          echo "Thank You! Your message has been sent.";
      } else {
          // Set a 500 (internal server error) response code.
          http_response_code(500);
          echo "Oops! Something went wrong and we couldn't send your message.";
      }
  
  } else {
      // Not a POST request, set a 403 (forbidden) response code.
      http_response_code(403);
      echo "There was a problem with your submission, please try again.";
  }
  
  ?&gt;
  </code></pre>
  
  <p>It seems like the ajax validation is working -- but once I hit submit there's an error. I'll include the HTML below as well.</p>
  
  <pre><code>&lt;form id="ajax-contact" method="post" action="php/mailer.php" role="form"&gt;
                    &lt;div class="form-group"&gt;
                      &lt;label for="name"&gt;Your Name&lt;/label&gt;
                      &lt;input type="text" id="name" class="form-control" required&gt;
                    &lt;/div&gt;
                    &lt;div class="form-group"&gt;
                      &lt;label for="email"&gt;Email address&lt;/label&gt;
                      &lt;input type="email" id="email" class="form-control" required&gt;
                    &lt;/div&gt;
                    &lt;div class="form-group"&gt;
                      &lt;label for="message"&gt;Message&lt;/label&gt;
                      &lt;textarea id="message" rows="5" class="form-control" required&gt;&lt;/textarea&gt;
                    &lt;/div&gt;
                    &lt;button type="submit" class="btn"&gt;Submit&lt;/button&gt;
                  &lt;/form&gt;
  </code></pre>
  
  <p>Please let me know if you need more information -- or if I need to debug this more on my own. Thanks for your time!</p>
  
  <pre><code>$(function() {
  
  // Get the form.
  var form = $('#ajax-contact');
  
  // Get the messages div.
  var formMessages = $('#form-messages');
  
  // Set up an event listener for the contact form.
  $(form).submit(function(e) {
      // Stop the browser from submitting the form.
      e.preventDefault();
  
      // Serialize the form data.
      var formData = $(form).serialize();
  
      // Submit the form using AJAX.
      $.ajax({
          type: 'POST',
          url: $(form).attr('action'),
          data: formData
      })
      .done(function(response) {
          // Make sure that the formMessages div has the 'success' class.
          $(formMessages).removeClass('error');
          $(formMessages).addClass('success');
  
          // Set the message text.
          $(formMessages).text(response);
  
          // Clear the form.
          $('#name').val('');
          $('#email').val('');
          $('#message').val('');
      })
      .fail(function(data) {
          // Make sure that the formMessages div has the 'error' class.
          $(formMessages).removeClass('success');
          $(formMessages).addClass('error');
  
          // Set the message text.
          if (data.responseText !== '') {
              $(formMessages).text(data.responseText);
          } else {
              $(formMessages).text('Oops! An error occured and your message           could not be sent.');
          }
      });
  
      });
  
      });
  </code></pre>
  
  <p>I've added the ajax. The first time I tested the page with the HTML corrected, it did exactly what I wanted. Now, however, the "Success" message will be shown on a new page after I click "Submit".</p>