[php] How can I check if a user is logged-in in php?

In file Login.html:

<html>
<head>
  <meta charset="utf-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
  <title>Login Form</title>
</head>
<body>
  <section class="container">
    <div class="login">
      <h1>Login</h1>
      <form method="post" action="login.php">
        <p><input type="text" name="username" value="" placeholder="Username"></p>
        <p><input type="password" name="password" value="" placeholder="Password"></p>

        <p class="submit"><input type="submit" name="commit" value="Login"></p>
      </form>
    </div>
</body>
</html>

In file Login.php:

<?php
    $host="localhost"; // Host name
    $username=""; // MySQL username
    $password=""; // MySQL password
    $db_name=""; // Database name
    $tbl_name="members"; // Table name

    // Connect to the server and select a database.
    mysql_connect("$host", "$username", "$password") or die("cannot connect");
    mysql_select_db("$db_name") or die("cannot select DB");

    // Username and password sent from the form
    $username = $_POST['username'];
    $password = $_POST['password'];

    // To protect MySQL injection (more detail about MySQL injection)
    $username = stripslashes($username);
    $password = stripslashes($password);
    $username = mysql_real_escape_string($username);
    $password = mysql_real_escape_string($password);
    $sql = "SELECT * FROM $tbl_name WHERE username='$username' and password='$password'";
    $result = mysql_query($sql);

    // Mysql_num_row is counting the table rows
    $count=mysql_num_rows($result);

    // If the result matched $username and $password, the table row must be one row
    if($count == 1){
        session_start();
        $_SESSION['loggedin'] = true;
        $_SESSION['username'] = $username;
    }

In file Member.php:

session_start();
if (isset($_SESSION['loggedin']) && $_SESSION['loggedin'] == true) {
    echo "Welcome to the member's area, " . $_SESSION['username'] . "!";
}
else {
    echo "Please log in first to see this page.";
}

In MySQL:

CREATE TABLE `members` (
    `id` int(4) NOT NULL auto_increment,
    `username` varchar(65) NOT NULL default '',
    `password` varchar(65) NOT NULL default '',
    PRIMARY KEY (`id`)
) TYPE=MyISAM AUTO_INCREMENT=2 ;

In file Register.html:

<html>
    <head>
        <title>Sign-Up</title>
    </head>

    <body id="body-color">
        <div id="Sign-Up">
            <fieldset style="width:30%"><legend>Registration Form</legend>
                <table border="0">
                    <form method="POST" action="register.php">
                        <tr>
                            <td>UserName</td><td> <input type="text" name="username"></td>
                        </tr>

                        <tr>
                            <td>Password</td><td> <input type="password" name="password"></td>
                        </tr>

                        <tr>
                            <td><input id="button" type="submit" name="submit" value="Sign-Up"></td>
                        </tr>
                    </form>
                </table>
            </fieldset>
        </div>
    </body>
</html>

In file Register.php:

<?php
    define('DB_HOST', '');
    define('DB_NAME', '');
    define('DB_USER','');
    define('DB_PASSWORD', '');

    $con = mysql_connect(DB_HOST, DB_USER, DB_PASSWORD) or die("Failed to connect to MySQL: " . mysql_error());
    $db = mysql_select_db(DB_NAME, $con) or die("Failed to connect to MySQL: " . mysql_error());

    $userName = $_POST['username'];
    $password = $_POST['password'];
    $query = "INSERT INTO members (username,password) VALUES ('$userName', '$password')";
    $data = mysql_query ($query) or die(mysql_error());
    if($data)
    {
        echo "Your registration is completed...";
    }
    else
    {
        echo "Unknown Error!"
    }

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