Error Too Many Redirects - php

Question

Trying to create my server.php script, so everythning was fine, till now. I wanted to prevent form resubmission and added header('location: index.php'); to my script. And then I faced the problem:ERR_TOO_MANY_REDIRECTS. And as many of you already understand my database was full of a junk. So, here is my code:
<?php
$username = $email = $password = "";
$usernameErr = $emailErr = $passwordErr = "";
$servername = 'localhost';
$serveruser = 'root';
$serverpassword = 'root';
$db = 'example';
$conn = new mysqli($servername, $serveruser, $serverpassword, $db);
if($conn->connect_error){
die("Connection failed: " . $conn->connect_error);
}
if(isset($_POST['register'])) {
$username = mysqli_real_escape_string($conn,$_POST['username']);
$email = mysqli_real_escape_string($conn,$_POST['email']);
$password = mysqli_real_escape_string($conn,$_POST['password']);
if(empty($username)) {
$usernameErr = "Username is required";
} else {
$username = test_input($_POST["username"]);
if(!preg_match("/^[a-zA-z ]*$/", $username)){
$usernameErr = "Only letters and whitespaces allowed";
}
}
if(empty($email)) {
$emailErr = "Email is required";
} else {
$email = test_input($_POST["email"]);
if(!filter_var($email, FILTER_VALIDATE_EMAIL)) {
$emailErr = "Wrong email format";
}
}
if(empty($password)) {
$passwordErr = "Password required";
} else {
$password = test_input($_POST["password"]);
}
}
if ($usernameErr == "" && $emailErr == "" && $passwordErr == "") {
$sql = "INSERT INTO users (username, email, password)
VALUES('$username','$email','$password')";
if($conn->query($sql) === TRUE) {
} else {
echo "Error: " . $sql . "<br>" . $conn->error;
}
header("location: index.php");
}
function test_input($data) {
$data = trim($data);
$data = htmlspecialchars($data);
$data = stripslashes($data);
return $data;
}
?>

Answer 1

To prevent TOO MANY REDIRECT put this code
if ($usernameErr == "" && $emailErr == "" && $passwordErr == "") {
$sql = "INSERT INTO users (username, email, password)
VALUES('$username','$email','$password')";
if($conn->query($sql) === TRUE) {
} else {
echo "Error: " . $sql . "<br>" . $conn->error;
}
header("location: index.php");
}
within
if(isset($_POST['register'])) { //.................}
this block after checking errors
And to prevent re-submission of form use accepted answer on this question
Preventing form resubmission

Answer 2

You can do couple of ways to stop this:
1) You can write either of these, unset($_POST['register']); or $_POST = array(); just before header('location:index.php');so it will not pass through if(isset($_POST['register'])) condition and so it will not go in infinite loop.
2) Or use full URL in header like this: header("location: mydomain.com/index.php"); It will stop infinite loop too.