Redirect to index.php after database validation through php

Question

I have made a signup/ Registration Page for a sample website but even after adding the header line to redirect to another page, this page just goes blank. This is working properly though when I run it locally on my machine. But as soon as I try to run in 000webhost this issue rises up. for example, the project start with the index page and then redirect to this sign up page, but as soon as the signup data is filled it should move back to the login page

<?php
    require_once "config.php";

    $username = $password = $confirm_password = "";
    $username_err = $password_err = $confirm_password_err = "";

    if($_SERVER["REQUEST_METHOD"] == "POST"){

        if(empty(trim($_POST["username"]))){
            $username_err = "Please enter a username.";
        } elseif(!preg_match('/^[a-zA-Z0-9_]+$/', trim($_POST["username"]))){
            $username_err = "Username can only contain letters, numbers, and underscores.";
        } else{
            $sql = "SELECT id FROM users WHERE username = ?";

            if($stmt = mysqli_prepare($link, $sql)){
                mysqli_stmt_bind_param($stmt, "s", $param_username);

                $param_username = trim($_POST["username"]);

                if(mysqli_stmt_execute($stmt)){
                    mysqli_stmt_store_result($stmt);

                    if(mysqli_stmt_num_rows($stmt) == 1){
                        $username_err = "This username is already taken.";
                    } else{
                        $username = trim($_POST["username"]);
                    }
                } else{
                    echo "Some issue occurred, please try again in some time";
                }

                mysqli_stmt_close($stmt);
            }
        }

        if(empty(trim($_POST["password"]))){
            $password_err = "Please enter a password.";
        } elseif(strlen(trim($_POST["password"])) < 6){
            $password_err = "Password must have atleast 6 characters.";
        } else{
            $password = trim($_POST["password"]);
        }

        if(empty(trim($_POST["confirm_password"]))){
            $confirm_password_err = "Please confirm password.";
        } else{
            $confirm_password = trim($_POST["confirm_password"]);
            if(empty($password_err) && ($password != $confirm_password)){
                $confirm_password_err = "Password did not match.";
            }
        }

        if(empty($username_err) && empty($password_err) && empty($confirm_password_err)){

            $sql = "INSERT INTO users (username, password) VALUES (?, ?)";

            if($stmt = mysqli_prepare($link, $sql)){
                mysqli_stmt_bind_param($stmt, "ss", $param_username, $param_password);

                $param_username = $username;
                $param_password = password_hash($password, PASSWORD_DEFAULT); // Creates a password hash

                if(mysqli_stmt_execute($stmt)){
                    **header("location: index.php");
                    exit;**
                } else{
                    echo "Some issue occured, please try again in some time";
                }

                mysqli_stmt_close($stmt);
            }
        }

        mysqli_close($link);
    }
?>