Try this:
<?php
$sidebar_options = array();
$sidebars = $GLOBALS['wp_registered_sidebars'];
foreach ( $sidebars as $sidebar ){
$sidebar_options[] = array(
'name' => $sidebar['name'],
'value' => $sidebar['id']
);
}
?>
This prints out( with my registered sidebars ):
Array
(
[0] => Array
(
[name] => Language Menu
[value] => sidebar-1
)
[1] => Array
(
[name] => Primary Sidebar
[value] => primary-sidebar
)
[2] => Array
(
[name] => Secondary Sidebar
[value] => secondary-sidebar
)
[3] => Array
(
[name] => Footer left box
[value] => footer_box_left
)
[4] => Array
(
[name] => Footer middle box
[value] => footer_box_middle
)
[5] => Array
(
[name] => Footer right box
[value] => footer_box_right
)
)
In a select tag this becomes:
<html>
<select>
<?php foreach( $sidebar_options as $option ){ ?>
<option value="<?php echo $option['value']; ?>"><?php echo $option['name']; ?></option>
<?php } ?>
</select>
</html>
Hope it helps!