<form id="login" action="process_login.php" type="POST" >
Username: <input type="text" id="username" />
Password: <input type="password" id="password" />
<input type="Submit" value="Login" />
</form>

In process_login.ph file...

$uname = $_POST['username']; // ID of username field in form will be username
$pswd = $_POST['password'];  // ID of password field in form will be password

$sql  = "SELECT username from users where username='$uname' AND password ='$pswd'"; // make sure the column names match the fields in your database

$result = mysql_query($sql);
$row = mysql_fetch_row($result); // will return number of match rows

if ($row) {
    header("Location: /successful.html"); // redirect the user to whichever page
  } else {
    header("Location: /failed.html");
  }