Showing all category and subcategory in magento

 <select class="select" id="option" value="option" name="option" onchange="short_cat(this.value)">
<option value="*">Select Category</option>
<?php $_helper = Mage::helper('catalog/category') ;
$_categories = $_helper->getStoreCategories();
$currentCategory = Mage::registry('current_category') ;

if (count($_categories) > 0): ?>
<?php foreach($_categories as $_category):
$selected='';
if($_REQUEST['catid']==$_category->getId())
{
$selected='selected';
}
?>
<option value="<?php echo $_category->getId() ?>"<?php echo $selected ?>><?php echo $_category->getName() ?></option>
            <?php $_category = Mage::getModel('catalog/category')->load($_category->getId()) ?>
            <?php $_subcategories = $_category->getChildrenCategories() ?>
            <?php if (count($_subcategories) > 0): ?>
                  <?php foreach($_subcategories as $_subcategory): ?>
                  <?php if($_REQUEST['catid']==$_subcategory->getId()){$selected='selected';}?>
                   <option value="<?php echo $_subcategory->getId() ?>"<?php echo $selected ?>>&nbsp;&nbsp;<?php echo $_subcategory->getName() ?></option>
                   <?php $_subcategory = Mage::getModel('catalog/category')->load($_subcategory->getId()) ?>
                   <?php $_sub_subcategories = $_subcategory->getChildrenCategories() ?>
                   <?php if (count($_sub_subcategories) > 0): ?>
                      <?php foreach($_sub_subcategories as $_sub_subcategory): ?>
                      <?php if($_REQUEST['catid']==$_sub_subcategory->getId()){$selected='selected';}?>
                          <option value="<?php echo $_sub_subcategory->getId() ?>"<?php echo $selected ?>>&nbsp;&nbsp;&nbsp;&nbsp;<?php echo $_sub_subcategory->getName() ?></option>
                      <?php endforeach; ?>          
                   <?php endif; ?>
                  <?php endforeach; ?>
            <?php endif; ?>  
<?php endforeach; ?>
<?php endif; ?>
</select>