page getting redirected to wrong page - php

Question

I have a simple login form that allows a user to login, although the form is working fine, but there is one condition where i wish to redirect it to someother page, instead of regular index page.
piece of code where i am facing issue is
if($spid=="")
{
header('Location:index.php');
}
else
{
header('Location:new.php?id=$spid');
}
the issue is even if the $spid has a value it is getting redirected to index.php page. can anyone please tell why this is happening
the whole code from which the above code has been extracted is
<?php
$spid=$_GET['spid'];
$emailErr = $pswrdErr = $loginErr = "";
$email = $password = "";
if ($_SERVER["REQUEST_METHOD"] == "POST") {
if (empty($_POST["email"])) {
$emailErr = "Email is required";
}
else {
$email = test_input($_POST["email"]);
if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
$emailErr = "Invalid email format";
}
}
if (empty($_POST["password"])) {
$pswrdErr = "password is required";
}
else {
$password = test_input($_POST["password"]);
}
$sql = "SELECT * FROM usertable where email='".$email."' and password='".$password."' ";
$result = mysqli_query($con, $sql);
if (mysqli_num_rows($result) > 0) {
while($row = mysqli_fetch_assoc($result)) {
$_SESSION['loggedin'] = true;
$_SESSION['email'] = $email;
if($spid=="")
{
header('Location:index.php');
}
else
{
header('Location:new.php?id=$spid');
}
}
}
else {
$loginErr = "Invalid Credentials";
}
}
function test_input($data) {
$data = stripslashes($data);
$data = htmlspecialchars($data);
return $data;
}
?>
<form action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]);?>" method="POST" role="form">
<input type="text" class="form-control" name="email">
<input type="password" class="form-control" name="password">
</form>

Answer 1

As you said you are getting value in $spid=$_GET['spid']; So instead of following above mentioned method take your $spid as hidden i/p text inside form and then pass it in your code as
$finalspid=$_POST["spid"]
and then put your if else condition according to $finalspid

Answer 2

Try this:
if(!isset($spid) && $spid==''){
header('Location:index.php');
} else {
header("Location:new.php?id=".$spid);
}