Why Multiple check Radio button . I want To Single Checked in from

My Code I want to single checked. but It is multiple check . How can i single selection in this types of situation ?

This is my code

<?php
$i=5;

for($i=1; $i<6; $i++)
{
   ?>
<form name="form" id="myform" class="frmclass">
   <input type="radio" name="rdo" value="<?php echo $i; ?>"> <?php echo $i; ?> <br>
</form>

<?php }

?>