if (!empty($_POST)) ...

if (!empty($_POST_01))...  // regForm
  and
if (!empty($_POST_02))...  //loginForm

if ((!empty($_POST_01)) And $x = 0)...

<form action="login.php" method="post">
    <input type="text" name="user">
    <input type="password" name="password">
    <input type="submit" value="Login">
</form>

<br />

<form action="register.php" method="post">
    <input type="text" name="user">
    <input type="password" name="password">
    <input type="submit" value="Register">
</form>

<form action="doStuff.php" method="post">
    <input type="text" name="user">
    <input type="password" name="password">
    <input type="hidden" name="action" value="login">
    <input type="submit" value="Login">
</form>

<br />

<form action="doStuff.php" method="post">
    <input type="text" name="user">
    <input type="password" name="password">
    <input type="hidden" name="action" value="register">
    <input type="submit" value="Register">
</form>

switch($_POST['submit']) {
    case 'login':
    //...
    break;
    case 'register':
    //...
    break;
}

if(isset($_POST['username']) && trim($_POST['username']) != "" && isset($_POST['password']) && trim($_POST['password']) != ""){
//login
} else {
//register
}