It doesn't matter...The important part is:
if($v['radio_button']==1) echo " selected ";
so if you are echoing your input field code:
<?php
echo '<input type="radio" name="radio_button" value="1"';
if($v['radio_button']==1) echo " selected ";echo '>';
echo '<input type="radio" name="radio_button" value="2"';
if($v['radio_button']==2) echo " selected ";echo '>';
etc
?>
Its not the prettiest way of doing it.