title: $_POST is not working in ajax form submit?
body: <p>When I try to check user input name is already exist by ajax form submit !But it only get <code>Undefined index: username</code> in <code>sessions.php</code> ,what is missing ?</p>
  
  <pre><code>&lt;form action="" method="POST" id="saveuser" enctype="multipart/form-data"&gt;
  &lt;input type="text" name="username"&gt;&lt;br&gt;
  &lt;input type="password" name="pass"&gt;&lt;br&gt;
  &lt;input type="file" name="fileupload"&gt;&lt;br&gt;
  &lt;input type="submit" name="submit" value="Confirm" id="confirm"&gt;
  &lt;/form&gt;
  &lt;script type="text/javascript"&gt;
      $('#confirm').click(function(e){
          e.preventDefault();
        $.ajax({
              type:'POST',
              url :"sessions.php",
              data:$("#saveuser").serialize(),
              contentType : false,
              processData: false,            
              success: function(d){
                  console.log(d);//[error] :Undefined index: username 
              }
          });
      });
  &lt;/script&gt;
  </code></pre>
  
  <p>sessions.php</p>
  
  <pre><code>&lt;?php
  $exist = "david";
  if($_POST['username'] == $exist){
      echo json_encode("Already exist");
  }
  else{
      echo json_encode("You can succesfully add");
  }
  ?&gt;
  </code></pre>