How do I authenticate a user in PHP / MySQL?

Question

So recently I learned how to properly add a username and password to a database. My database is usersys, and the table storing user information is called userdb. The table h

Answer 1

One way to do it (DISCLAIMER: not necessarily best-practice):

$result = mysql_query("SELECT id FROM userdb WHERE username='$esclcusername' AND  password='$escpassword'") or die(mysql_error());
$row = mysql_fetch_array( $result );
$id = (int)$row['id'];
if($id > 0) {
    //log in the user
    session_start();
    $_SESSION['userId'] = $id;
    $_SESSION['username'] = $displayname;
}

... and on pages that require authentication:

session_start();
if(!isset($_SESSION['userId'])) {
    die('You need to be logged in!!!');
} else {
    echo 'Welcome ' . $_SESSION['username'];
}

Read more about PHP sessions.