PHP: this webpage has a redirect loop - php

Question

I know this error is very common and plenty of solution are mentioned but for some reason I cant get it working.
This is my index.php file which is the log in page for users
<?php
session_start();
if (!(isset($_SESSION['username']) && $_SESSION['username'] != '')) {
header ("Location: index.php");
}
?>
...
...
<form name='log' action='loginproc.php' method='POST'>
<input type="text" placeholder="Username" name="username">
<input type="password" placeholder="Password" name="password">
<input type='submit' value='Login' onclick="return check()">
</form>
This is the loginproc.php file
<?php
require 'config.php';
require 'core.inc.php';
if(isset($_POST['username'])&&isset($_POST['password'])) {
$username = $_POST['username'];
$password = $_POST['password'];
if(!empty($username) && !empty($password)) {
// $md5pwd = md5($password);
$query = "SELECT username FROM `login` WHERE `username` = '$username' and `password` = '$password';";
//echo $query;
//$query_run = mysqli_query($query);
//echo $query_run;
if($query_run = mysqli_query($conn,$query)) {
//echo $query_run;
$query_num_rows = mysqli_num_rows($query_run);
if($query_num_rows==0) {
phpAlert( "Invalid USERNAME or PASSWORD combination" );
echo '<script type="text/javascript">window.location = "index.php"</script>';
exit();
//echo '<p> Invalid USERNAME or PASSWORD combination <br /><br /> Click here to Login ';
}
else if($query_num_rows==1) {
//echo 'Hello';
$_SESSION['username']=$username;
header('Location: main.php');
}
}
}
else {
echo 'You Must supply a username and password';
}
}
function phpAlert($msg) {
echo '<script type="text/javascript">alert("' . $msg . '")</script>';
}
?>
The core.inc.php file
<?php
ob_start();
session_start();
function loggedin() {
if(isset($_SESSION['username'])&& !empty($_SESSION['username'])) {
return true;
}
else {
return false;
}
}
?>

Answer 1

you need to add an extra parameter when redirecting to index.php.
<?php
session_start();
if (!(isset($_SESSION['username']) && $_SESSION['username'] != '')) {
if($_REQUEST['ftime']!=1)
{
header ("Location: index.php?ftime=1");
}
}
?>

Answer 2

You need to have the redirect go elsewhere for logged in users and bypass when not logged in:
<?php
session_start();
// if session username is set and valid, go to somewhere else
if(isset($_SESSION['username']) && !empty($_SESSION['username'])) {
// elswhere could be a profile page, or similar
header("Location: elsewhere.php");
// It is more accepted to exit after a header
exit;
}
?>
<!-- All other instances beside valid login will allow this to form to show-->
<form name='log' action='loginproc.php' method='POST'>
<input type="text" placeholder="Username" name="username">
<input type="password" placeholder="Password" name="password">
<input type='submit' value='Login' onclick="return check()">
</form>