PHP - How Do I Insert File Contents Into Response?

I am making a simple login with jQuery ajax but I have a little problem. I get no response from my loginizer.php . 

My login page (index.php) contains the following code.

<form action="loginizer.php"  onsubmit="return submitForm();"  method="post" class="box">

<div id="error" ></div>

 <input id="matNo" name="matNo" class="input is-link is-rounded" type="text" placeholder="e.g. 2015/1/55852EE">
   <input id="password" name="password" class="input is-link is-rounded" type="password" placeholder="********">

 <button type="submit"  id="submit" name="submit" class="button is-link is-small">Login</button></form>

My loginizer php contains the code:

<?php
if(isset($_POST['submit'])){
require("db.php");
$table="Students";
$pUser=$_POST['matNo'];
$pPass=$_POST['password'];


$sql= $conn->prepare("SELECT * FROM  $table WHERE Email=?");
$sql->bind_param("s",$pUser);
$sql->execute();
$result = $sql->get_result();
$row = $result->fetch_assoc();
$user= $row['Email'];
$pass = $row['Password'];
$errMsg="Invalid Credentials";
$errMsg1="username doesn't exist";
$successMsg = "successLogin";
if($result->num_rows < 1)
{

echo json_encode($errMsg1);
exit;
}
elseif($pUser == $user && $pPass !== $pass)
{
echo json_encode($errMsg);
exit;
}
else{
echo json_encode($successMsg);
exit;
}
}
?>

Ans lastly myJquery.js file which contains:

function submitForm()
{

 var userid=$("#matNo").val();
 var pass=$("#password").val();
 if(userid!="" && pass!="")
 {

  $("#submit").html("Please Wait...");
  $.ajax
  ({
  type:'post',
  url:'loginizer.php',
  data:{
   matNo:userid,
   password:pass
  },
   dataType:'json',

  success:function(response) {

  if(response=="successLogin")
  {

    window.location.href="dash/home/";
  }
  else
  {

    $("#error").html(response);
    //alert("Wrong Details");
  }
 }

   });
 }

 else
 {
  alert("All details are required");
 }

 return false;
}

The issue is that I get blank response . If I also change the url in the jQuery/Ajax code to index.php, it comes back with a response consisting of the html of the index.php. If I remove the dataType: 'json', it's still no response.

Thanks for your help in advance!!!

A HTTP request is made to the server, Slim creates a Request and Response object, content in the Request is sent to another server using cURL via Guzzle, Guzzle returns its own Response,  and content from Guzzle's response must be returned by the original Slim response.

Would you recommend white-listing or black-listing response headers, and which specific headers?

Similarly, would you recommend white-listing or black-listing the request headers sent via cURL, and which specific headers?

Thanks

<?php
use Psr\Http\Message\ResponseInterface as Response;
use GuzzleHttp\Psr7\Response as CurlResponse;

class ApiResponder
{
    public function delete(Response $httpResponse, CurlResponse $curlResponse) {
        return $this->proxy($httpResponse, $curlResponse);
    }
    //other public methods...

    private function proxy(Response $httpResponse, CurlResponse $curlResponse) {
        foreach($this->getResponseHeaders($curlResponse) as $name=>$value) {
            $httpResponse=$httpResponse->withHeader($name, $value);
        }
        return $httpResponse->withBody($curlResponse->getBody())->withStatus($curlResponse->getStatusCode());
    }

    private function getResponseHeaders(Response $httpResponse):array
    {
        //Blacklist headers which should be returned to original client.  TBD whether I should whitelist headers instead.
        $blacklist=['Date'=>null, 'Server'=>null, 'X-Powered-By'=>null, 'Access-Control-Allow-Origin'=>null, 'Access-Control-Allow-Methods'=>null, 'Access-Control-Allow-Headers'=>null, 'Set-Cookie'=>null];
        return array_diff_key($curlResponse->getHeaders(), $blacklist);
    }

    /**
    * This method doesn't really exist in this class, but is just included to show which headers I am forwarding in the cURL request.
    */
    private function getRequestHeaders($clientRequest):array
    {
        $whitelist=['connection'=>null,'accept'=>null,'accept-encoding'=>null,'accept-language'=>null,'content-type'=>null,'content-length'=>null];
        return array_intersect_key($clientRequest->getHeaders(), $whitelist);
    }
}