require_once 'connect.inc.php';
require_once 'core.inc.php';

if(isset($_POST['username']) && isset($_POST['password'])){
    $username=$_POST['username'];
    $password=$_POST['password'];
    $password_hash=md5($password);
    if(!empty($username) && !empty($password)){
        $query="SELECT `id` FROM `login` WHERE `username`='$username' AND `password`='$password_hash'";
        if($query_run = mysqli_query($stat,$query)){
            $query_num_rows = mysqli_num_rows($query_run);
            if($query_num_rows == 0){
                echo 'You have entered wrong username or password !';
            }
            else if($query_num_rows == 1){
                function mysqli_result($res, $row, $field) {
                    $res->data_seek($row);
                    $datarow = $res->fetch_array();
                    return $datarow[$field];
                }
                $user_id= mysqli_result($query_run, 0,'id');
                $_SESSION['user_id'] = $user_id;
                header('Location: index.php');
            }
        }
        else
        {
            echo 'You have entered wrong username or password !';
        }
    }else {
        echo 'You must have to enter username and password';
    }
}
<form action="<?php echo $server; ?>" method="POST">
    Username:
    <input type="text" placeholder="Enter the userid" name="username">
    Password:
    <input type="password" placeholder="Enter your password" name="password">
    <input type="submit" value="Log in">

require_once 'core.inc.php';
require_once 'connect.inc.php';

if(get_status()){
$firstname=getinfo('firstname');
$lastname=getinfo('lastname');
echo 'You are logged in ,'.$firstname.' '.$lastname.'';
echo "<br><a href='logout.php'>Logout</a><br>";
}
else {
 include_once 'login.php';
}

session_start();
ob_start();
$server =$_SERVER['SCRIPT_NAME'];
@$ref_url=$_SERVER['HTTP_REFERER'];
function get_status(){
    if(isset($_SESSION['user_id']) && !empty($_SESSION['user_id'])){
        return true;
    }
    else
        return false;
}

function getinfo($field){
    $get_query="SELECT `$field` FROM `login` WHERE `id`='".$_SESSION['user_id']."'";
    require 'connect.inc.php';
    if($query_run=mysqli_query($stat,$get_query)){
            function mysqli_result($res, $row, $f) {
            $res->data_seek($row);
            $datarow = $res->fetch_array();
            return $datarow[$f];
        }
        if($query_result=mysqli_result($query_run,0,$field)){
            return $query_result;
        }
    }
}