first off, please do not downvote me as I need to learn! we all have to start from somewhere.

I am not asking anyone to write me a code but I just want to learn it in simple terms.

and the best way for me to learn is to edit my code so I can compare them.

here is my code:

<?php
if (isset ($_POST['email']))  {
//Connect to the database through our include 
include_once "config/connect.php";
$email = stripslashes($_POST['email']);
$email = strip_tags($email);
$email = mysqli_real_escape_string($db_conx, $email);
$password = preg_replace("[^A-Za-z0-9]", "", $_POST['password']); // filter everything but numbers and letters
$password = md5($password);
// Make query and then register all database data that -
// cannot be changed by member into SESSION variables.
// Data that you want member to be able to change -
// should never be set into a SESSION variable.
$sql = "SELECT * FROM members WHERE email='$email' AND password='$password'"; 
$query = mysqli_query($db_conx, $sql);
$login_check = mysqli_num_rows($query);
if($login_check > 0){ 
    while($row = mysqli_fetch_array($query, MYSQLI_ASSOC)){ 
        // Get member ID into a session variable
        $id = $row["id"];   
        session_register('id'); 
        $_SESSION['id'] = $id;
        // Get member username into a session variable
        $username = $row["username"];
        $email = $row["email"];
        $password = $row["password"];
        $firstname = $row["firstname"];
        $lastname = $row["lastname"];
                session_register('username'); 
        session_register('firstname');
        session_register('lastname');
        // Update last_log_date field for this member now
        $sql = "UPDATE members SET lastlogin=now() WHERE id='$id'";
        $query = mysqli_query($db_conx, $sql); 
        // Print success message here if all went well then exit the script
        header("location: members/index.php?id=$id"); 
        exit();
    } // close while
} else {
// Print login failure message to the user and link them back to your login page
header("location: login.php");
  exit();
}
}
?>

Thanks in advance

I am not asking anyone to write me a code, but I just want to learn it in simple terms. The best way for me to learn is to edit my code so I can compare them.

<?php
if (isset ($_POST['email']))  {
    //Connect to the database through our include 
    include_once "config/connect.php";
    $email = stripslashes($_POST['email']);
    $email = strip_tags($email);
    $email = mysqli_real_escape_string($db_conx, $email);
    $password = preg_replace("[^A-Za-z0-9]", "", $_POST['password']); // filter everything but numbers and letters
    $password = md5($password);
    // Make query and then register all database data that -
    // cannot be changed by member into SESSION variables.
    // Data that you want member to be able to change -
    // should never be set into a SESSION variable.
    $sql = "SELECT * FROM members WHERE email='$email' AND password='$password'"; 
    $query = mysqli_query($db_conx, $sql);
    $login_check = mysqli_num_rows($query);
    if($login_check > 0){ 
        while($row = mysqli_fetch_array($query, MYSQLI_ASSOC)){ 
            // Get member ID into a session variable
            $id = $row["id"];   
            session_register('id'); 
            $_SESSION['id'] = $id;
            // Get member username into a session variable
            $username = $row["username"];
            $email = $row["email"];
            $password = $row["password"];
            $firstname = $row["firstname"];
            $lastname = $row["lastname"];
                    session_register('username'); 
            session_register('firstname');
            session_register('lastname');
            // Update last_log_date field for this member now
            $sql = "UPDATE members SET lastlogin=now() WHERE id='$id'";
            $query = mysqli_query($db_conx, $sql); 
            // Print success message here if all went well then exit the script
            header("location: members/index.php?id=$id"); 
            exit();
        } // close while
    } else {
    // Print login failure message to the user and link them back to your login page
    header("location: login.php");
        exit();
    }
}
?>