Im definiing radio group:
<?php
    $a = new \Phalcon\Forms\Element\Radio('prepaytype_a', array('name' => 'prepaytype'));
    $a->setDefault(1);
    $a->setAttribute('checked', 'checked');
    $this->add($a);
    $b = new \Phalcon\Forms\Element\Radio('prepaytype_b', array('name' => 'prepaytype'));
    $b->setDefault(2);
    $this->add($b);but they gets rendered like that:
<input type="radio" checked="checked" value="1" name="prepaytype" id="prepaytype_a">
<input type="radio" checked="checked" value="2" name="prepaytype" id="prepaytype_b">
while only the first should be checked.