Допустим, есть такой тестовый код:
<?php
if ((isset($_POST['a'])) && ($_POST['a'] == 1)) header('Location: test2.php');
?><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Тест</title>
</head>
<body>
<form action="#form" method="post">
<input type="hidden" name="a" value="1">
<input type="submit">
</form>
</body>
</html>
