How to Login in php

connect.php'); //add connection variables// If the user isn't logged in, try to log them in

// Connect to the database
$dbc = mysqli_connect(DB_HOST, DB_USER, DB_PASSWORD, DB_NAME);

// Grab the user-entered log-in data
$user_username = mysqli_real_escape_string($dbc, trim($_POST['username']));
$user_password = mysqli_real_escape_string($dbc, trim($_POST['password']));

if(!empty($user_username)) {
if(!empty($user_password)) {
// Look up the username and password in the database
$query = "SELECT agent_id, username FROM hj_buses_agent WHERE username = '$user_username' AND password = '$user_password'";
$data = mysqli_query($dbc, $query);

if(mysqli_num_rows($data) == 1) {
// The log-in is OK so set the user ID and username session vars (and cookies), and redirect to the home page
$row = mysqli_fetch_array($data);
$_SESSION['agent_id'] = $row['agent_id'];
$_SESSION['username'] = $row['username'];
setcookie('agent_id', $row['agent_id']);
setcookie('username', $row['username']);
$home_url = 'http://' . $_SERVER['HTTP_HOST'] . dirname($_SERVER['PHP_SELF']) . '/index.php';
header('Location: ' . $home_url);
}
else {
// The username/password are incorrect so set an error message
$home_url = 'http://' . $_SERVER['HTTP_HOST'] . dirname($_SERVER['PHP_SELF']) . '/login.php?data=true';
header('Location: ' . $home_url);
}
}else

{
echo'';
}
}
else {
// The username/password weren't entered so set an error message
echo'';
} ?>