example php login

<?
	$action = isset($_POST['action']) ? $_POST['action']:'';
	if($action == 'validatelogin')
	{
		$DbHost     = "localhost";
		$DbDatabase = "DATABASENAME";
		$DbUser     = "USERNAME";
		$DbPassword = "PASSWORD";
		// --- PDO Info
		$dsn = 'mysql:host='.$DbHost.';dbname='.$DbDatabase;
		$DbOptions = array(PDO::MYSQL_ATTR_INIT_COMMAND => 'SET NAMES utf8',PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION);
		$DBH = new PDO($dsn, $DbUser, $DbPassword, $DbOptions);
		$password = isset($_POST['password'])?$_POST['password']:'';
		$username = isset($_POST['username'])?$_POST['username']:'';
		if($password == ''){die('Password cannot be blank!');}
		if($username == ''){die('Username cannot be blank!');}
		$qs = "SELECT COUNT(*) as `count` FROM `users` WHERE `username`=:username";
		$q = $DBH->prepare($qs);
		$q->bindValue(':username', (string)$username, PDO::PARAM_STR);
		$q->execute();
		$count = $q->fetch(PDO::FETCH_ASSOC)['count'];
		if($count > 0)
		{
			$qs = "SELECT `pwhash` FROM `users` WHERE `username`=:username LIMIT 1";
			$q = $DBH->prepare($qs);
			$q->bindValue(':username', (string)$username, PDO::PARAM_STR);
			$q->execute();
			$pwhash = $q->fetch(PDO::FETCH_ASSOC)['pwhash'];
			if (password_verify($password, $pwhash))
			{
				//Session stuff, redirect
				die('Login Granted');
			}
			else
			{
				die('Invalid Username/Password');
			}
		}
		else
		{
			die('Invalid Username/Password');
		}
	}
?>
<!doctype html>
<html>
	<head>
	</head>
	<body>
		<h4 id="result"><h4>
		username<br>
		<input type="text" id="username"/><br>
		password<br>
		<input type="password" id="password"/><br>
		<input type="button" value="login" onclick="ajaxLogin()"/><br>
		<script>
			function ajaxLogin()
			{
				var username = encodeURIComponent(document.getElementById('username').value);
				var password = encodeURIComponent(document.getElementById('password').value);
				var params = "action=validatelogin&username="+username+"&password="+password;
				var url = "<?=basename($_SERVER['PHP_SELF']);?>?time="+ (new Date()).getTime();
				var http = new XMLHttpRequest();
				http.open("POST", url, true);
				http.setRequestHeader("Content-type","application/x-www-form-urlencoded");
				http.onload = function()
				{
					if(http.readyState == 4 && http.status == 200)
					{
						var result = http.response;
						document.getElementById('result').innerHTML = result;
					}
				};
				http.send(params);

			}
		</script>
	</body>
</html>