PHP Login function

<?php

session_start();

$loginUsr = ('usernamehere'); /* Username for the page */
$loginPswd = ('passwordhere'); /* Password for the page */

$username = ($_POST['username']); /* Name of usernamefield on login page */
$password = ($_POST['password']); /* Name of password field on login page */

if ($username == $loginUsr && $password == $loginPswd) {
    $_SESSION['loggedin'] = 'true';
    header("Location: admin.php");
    exit();
} else {
    echo('Login failed');
    session_destroy();
}