If no output is done:
header('Location: index.php');
If your form handles errors:
include 'index.php';
If you just need a simple redirect:
<META HTTP-EQUIV="REFRESH" CONTENT="0; URL=index.php">
Perhaps a javascript redirect:
<script>window.location.href='index.php';</script>
Have frames?
<script>window.top.location.href='index.php';</script>