<?php if ($_GET['do'] == 'reg'): ?>
<b><div class="reg"></div></b>
...
<?php elseif ($_GET['do'] == 'sign'): ?>
GET=sign
...
<?php endif; ?><!DOCTYPE HTML>
<html>
<head>
<title>Firebase</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<link href="css/style.css" rel="stylesheet">
</head>
<body>
<?php new Register(); ?>
<form class="pure-form" method="post">
<h2>Registration</h2>
<input name="addlogin" placeholder="Type your login" type="text">
<br><br><br>
<input name="addpassword" placeholder="Type your password" type="password">
<br><br><br>
<input name="addemail" placeholder="Type your email address" type="text">
<br><br><br>
<button type="submit" name="submitter" class="pure-button pure-button-primary">
Register
</button>
<br>
<hr>
<br>
</form>
</body>
</html> вместо пробела<pre>text </pre>header('Location: http://site.ru?redirect=1');if( isset($_GET["redirect"]) )
$_POST = "asd";
echo $_POST; // выдаст "asd"$asd = "qwerty";
echo $asd [0] // выдаст "q"
echo $asd [2] // выдаст "e"
echo $asd [100] // выдаст пустоту