Accessing Viariable from Other PHP File - php

Question

I'm still starting to learn PHP and i don't know which part did i miss why i can't access the variable from other php file even though i include this in my index file.
Here's how i include the other php into my index file
index.php
<?php
include 'includes/signup.inc.php';
?>
With in this index.php I have this form:
<form id="form" action="includes/signup.inc.php" method="POST">
<div id="form1">
<label for="fname">First Name:</label>
<input type="text" name="fname"><span><?php echo $fnameerr?></span>
<br>
</div>
</form>
the variable $fnameerr is a variable from other php file inside includes folder named signup.inc.php if I only have this code inside signup.inc.php
<?php
include 'dbh.inc.php';
$fname = $lname = $email = $uid = $pw = "";
$fnameerr = $lnameerr = $emailerr = $uiderr = $pwerr = "";
this variable $fnameerr can be seen from index.php. But when i start to include this code:
<?php
include 'dbh.inc.php';
$fname = $lname = $email = $uid = $pw = "";
$fnameerr = $lnameerr = $emailerr = $uiderr = $pwerr = "";
if ($_SERVER['REQUEST_METHOD'] == "POST")
{
if (empty($_POST['fname']))
{
$fnameerr = "First Name Required";
}
else
{
$fname = cleandata($_POST['fname']);
}
if (empty($_POST['lname']))
{
$lnameerr = "Last Name Required";
}
else
{
$lname = cleandata($_POST['lname']);
}
if (empty($_POST['email']))
{
$emailerr = "Email Required";
}
else
{
$email = cleandata($_POST['email']);
}
if (empty($_POST['uid']))
{
$uiderr = "User ID Required";
}
else
{
$uid = cleandata($_POST['uid']);
}
if (empty($_POST['pw']))
{
$pwerr = "Password Required";
}
else
{
$pw = cleandata($_POST['pw']);
}
}
function cleandata($data)
{
$data = trim($data);
$data = stripslashes($data);
$data = htmlspecialchars($data);
return $data;
}
if (!empty($fname) || !empty($lname) || !empty($email) || !empty($uid) || !empty($pw))
{
$sql = "INSERT INTO userlist(fname, lname, email, uid, pw) VALUES('$fname', '$lname', '$email', '$uid', '$pw');";
}
mysqli_query($conn, $sql);
header("location:../index.php");
?>
I can't access my index.php instead i am redirected to dashboard.