The main difference is weight and the speed of the bike.
Mountain bikes are usually heavier, partly because of the shocks and gears.
Road bikes have thin tires, and tall frames, and well, meant for the road.
You can adjust to the road bikes after riding for a mile.
In your HTML form, give names to the fields you want to "GET".
<form action="myPHP.php" method="get">
<input name="user" type="text">
<input name="pass type="text">
<input type="submit">
</form>
In your PHP assign those values to a variable:
$username = $_GET['user'];
$password = $_GET['pass']...