<?php

    include 'dbconfig.php';
    $username = $_POST['username'];
    $password = $_POST['password'];
    $sql = "SELECT * FROM login WHERE username = '$username' AND password= '$password' ";
    $result = $conn->query($sql);

    if ($result->num_rows > 0) {
        // output data of each row
        while($row = $result->fetch_assoc()) {
            $_SESSION["userid"] = $row['user'];
            echo $_SESSION["userid"];
        }
    } else {
        echo "wrong";
    }
    $conn->close();
    ?>

function login() {

    var username = $("#username").val();
    var password = $("#password").val();
    if (username == "" || username == null || username == undefined || password == "" || password == null || password == undefined) {
        $('#foremptyvalue').show();
    } else {
        $('#foremptyvalue').hide();
        $('#loader').show();
        jQuery.ajax({
            url: baseurl + "login.php"
            , data: 'username=' + username + '&password=' + password
            , type: "POST"
            , success: function (response) {
                response = $.trim(response);
                if (response == "wrong") {
                    $('#loader').hide();
                    $('#forwronginput').show();
                    $("#username").val('');
                    $("#password").val('');
                } else {
                    $('#loader').hide();
                    $('#forwronginput').hide();
                    location.href = "account.php?id=" + response;
                }

            }
            , error: function () {}
        });
    }

}

<?php
session_start();
echo session_id();
$session = $_SESSION["userid"];
?>