<form method="POST" action="<?php echo $_SERVER['PHP_SELF']; ?>">
<input type="password" name="pwd" id="pwd"><input type="submit">
</form>

<?php
$pwd = $_GET['pwd']; // you should sanitize this, but I have beer to drink
if($pwd) { // this is just checking if the $pwd variable is set
	if($pwd == "trex") { // check if the password is trex
		header(location: 'https://www.youtube.com/watch?v=BXpgIQV0-R0'); // send them to another url
	} else { // otherwise tell them the pass is wrong
		echo "Wrong password!";
	}
}
?>