I have a PHP website. I want to create a signup-login process. I have completed the signup process but when I log in I need to do some verification only after that it should redirect to the page where I want it to redirect.

I have the following pages:

index.php,
login.php,
handleLogin.php, and
blog.php

I want to make the validation like this when we log in from index.php or any other page but not from the blog page, there will be a link named login in the header, and when someone wants to click from there the login page will appear and after logged in, it will redirect to index.php. But when we click from the comment on the blog page, firstly it will redirect to the login page and when logged in is successful it will redirect to the same blog page.

Following are the HTML codes of the login page:

<div class="login_form">
        <form action="handleLogin.php" method="POST" class="login">
            <div class="mb-3">
                <label for="exampleInputEmail1" class="form-label">Email address</label>
                <input type="email" name="vEmail" class="form-control" id="exampleInputEmail1" aria-describedby="emailHelp">
            </div>

            <div class="mb-3">
                <label for="exampleInputPassword1" class="form-label">Password</label>
                <input type="password" name="vPass" class="form-control" id="exampleInputPassword1">
                <div id="emailHelp" class="form-text">We'll never share your email & password with anyone else.</div>
            </div>

            <button type="submit" class="btn btn-primary mb-3">Log in</button>

            <div class="create_account">
                <label for="exampleInputEmail1" class="form-label">Don’t have an account? <a href="signup.php"
                        class="signup_link">Sign up</a></label>
            </div>
        </form>
    </div>

Following are the HTML codes of the handleLogin page:

<?php
    // session_start ();
    // $bid = $_GET['b'];
    $bid = '0';
   
        $showAlert = "false";
        if($_SERVER["REQUEST_METHOD"]=="POST"){
            include '../partials/_dbconnect.php';

            $viewerEmail = $_POST['vEmail'];
            $viewerPass = $_POST['vPass'];

            $loginSql = "SELECT * FROM `viewers` WHERE `viewer_email`='$viewerEmail'";
            $loginResult = mysqli_query($conn, $loginSql);
            $loginNumRows = mysqli_num_rows($loginResult);

            if ($loginNumRows==1) {
                $showAlert = "Password not match";

                $loginRow = mysqli_fetch_assoc($loginResult);
                if (PASSWORD_VERIFY($viewerPass, $loginRow['viewer_pass'])) {

                    session_start();
                    $_SESSION['loggedin'] = true;
                    $_SESSION['vid'] = $row['viewer_id'];
                    $_SESSION['viewer_email'] = $row['viewer_email'];
                    $_SESSION['viewer_name'] = $row['viewer_name'];

                    // start
                    $showAlert = "No authority";
                    if (isset($_SESSION['loggedin']) && $_SESSION['loggedin']==true) {
                        // header("Location: ../mains/blog?logininsuccess=true&b=$bid");
                        // exist();
                        if (($bid!=0) && ($bid!=NULL)) {
                            header("Location: ../mains/blog?logininsuccess=true&b=$bid");
                            exist();
                        }
                        else {
                            // header("Location: ../index");
                            // exist();
                            $showAlert = "No bid";
                            if (isset($_SESSION['loggedin']) && $_SESSION['loggedin']==true) {
                                
                                $showAlert = "Unknown bid";
                                if (($bid=0) && ($bid=NULL) && ($bid='')) {
                                    header("Location: ../index?no_bid");
                                    exist();
                                }
                                else {
                                    header("Location: login?loginsuccess=false&error=$showAlert");
                                }
                            }
                            else {
                                header("Location: login?loginsuccess=false&error=$showAlert");
                            }
                        }
                    }
                    else {
                        header("Location: login?loginsuccess=false&error=$showAlert");
                    }
                    // end
                }
            }
            else {
                $showAlert = "Email not exist";
            }
            header("Location: login?loginsuccess=false&error=$showAlert");
        }

?>

The result I am getting is as follows:

loginsuccess=false&error=Unknown%20bid

The result I am expected is as follows:

When $bid='0' or $bid='NULL' or $bid='' or there is no $bid then after logged in is successful it should redirect to index.php, but if $bid='5' then it should redirect to http://localhost/ows-backup-30-7-24/mains/blog?b=5