The boolean is set to The third argument.

<?php echo radiobutton_tag('status', 'value1', true) ?>
<?php echo radiobutton_tag('status', 'value2', false) ?>

It write like this.

<?php echo radiobutton_tag("status", "value1", $sf_params->get("status") == 'value1'); ?>
<?php echo radiobutton_tag("status", "value2", $sf_params->get("status") == 'value2'); ?>