HTML - Problema con el login de mi web (html, php y mysql)

<form method="post" action="paginas/login/entrar.php">

<input type="text" name="username" id="username" placeholder="Nombre de usuario" required>

<input type="password" name="password" id="password" placeholder="Contraseña" required>

<input type="submit" name="login" value="¡Conectarse!">

<br/><a href="">¿Has perdido tu contraseña?</a>

</form>

<?php

	$usuario = $_POST['username'];

	$pass = $_POST['password'];

	if(empty($usuario) || empty($pass)){

	header("Location: ../../index.html");

	exit();

}

	mysql_connect('localhost','root','ascent') or die("Error al conectar " . mysql_error());

	mysql_select_db('auth') or die ("Error al seleccionar la Base de datos: " . mysql_error());

	$result = mysql_query("SELECT * from auth where Username='$usuario' AND sha_pass_hash='$pass'");

		//    mysql_query("SELECT usuario, password FROM empresas WHERE usuario='$usuario' and password='$password'");

	if($row = mysql_fetch_array($result)){

	if($row['sha_pass_hash'] == $pass){

	session_start();

	$_SESSION['username'] = $usuario;

	header("Location: contenido.php");

	}else{

	header("Location: ../../index.html");

	exit();

	}

	}else{

	header("Location: ../../index.html");

	exit();

	}

?>