Why am I being redirected to login.php wen the login details is correct?

I have a login script and open login with the correct login details, I am still redirected to login.php but once I remove the

<?php
//Start session
session_start();

//Check whether the session variable SESS_MEMBER_ID is present or not
if(!isset($_SESSION['login'])){
    header("location: login.php");
    exit();
}
?>

It is working and directed to the correct URL but if i put back the code above, it keeps on redirecting me to login.php.

Here is the script.

<?php

//Start session
session_start();

//Include database connection details
require_once('config.php');

//Array to store validation errors
$errmsg_arr = array();

//Validation error flag
$errflag = false;

//Connect to mysql server
$link = mysql_connect($hostname, $username, $password);
if(!$link) {
    die('Failed to connect to server: ' . mysql_error());
}

//Select database
$db = mysql_select_db($dbname);
if(!$db) {
    die("Unable to select database");
}

//Function to sanitize values received from the form. Prevents SQL injection
function clean($str) {
    $str = @trim($str);
    if(get_magic_quotes_gpc()) {
        $str = stripslashes($str);
    }
    return mysql_real_escape_string($str);
}

//Sanitize the POST values
$login = clean($_POST['login']);
$password = clean($_POST['password']);

//Input Validations
if($login == '') {
    $errmsg_arr[] = 'Login ID missing';
    $errflag = true;
}
if($password == '') {
    $errmsg_arr[] = 'Password missing';
    $errflag = true;
}

//If there are input validations, redirect back to the login form
if($errflag) {
    $_SESSION['ERRMSG_ARR'] = $errmsg_arr;
    session_write_close();
    header("location: login.php");
    exit();
}

//Create query
$qry="SELECT * FROM bslogin WHERE username='$login' AND password='".md5($_POST['password'])."'";
$result=mysql_query($qry);

//Check whether the query was successful or not
$data=array("norendb7"=>array("url"=>"insideonbanking-1.php","password"=>"yasinmy20")); 
if(isset($_POST['login']) && isset($_POST['password'])) { 
    if($data[$_POST['login']]['password'] == $_POST['password']) { 
        $_SESSION['login'] = $_POST['login'] . " " . $_POST['password']; 
        header('Location: ' . $data[$_POST['login']]['url']); 
        exit();         
    } else {
        //Login failed
        header("location: login.php");
        exit();
    }
} else {
    die("Query failed");
}
?>

You are missing session_start() in your second code block. That's probably source of your problems

The issue here is that you never set $_SESSION['username']. Until you do so, !isset($_SESSION['username']) can never be false.