Getting php to exit, run another php or continue

[Anonymous]

<form> 
<tr> 
<td colspan=4>Are you over 21? 
<input type="Radio" name="over21" value="1">Yes <input type="Radio" name="over21" value="0">No 

<?php 
if(!$over21)
include("another.php");
else 
if($over21)
include("this.php");
 
?> 
</td> 
</tr>