Processing form-required-fields - php

Question

I can´t figure out how to edit this code in order to get all cotegories (dropdown-list-fields) being required fields. This script only validates up to 2 dropdown-list-fields before being processed and I have up to 4 categories dropdown-list-fields. Any ideas?
<!-- category select -->
<div class="form-group">
<div class="col-md-12">
<?= FORM::label('category', __('Category'), array('for'=>'category'))?>
<div id="category-chained" class="row <?=($id_category === NULL) ? NULL : 'hidden'?>"
data-apiurl="<?=Route::url('api', array('version'=>'v1', 'format'=>'json', 'controller'=>'categories'))?>"
data-price0="<?=i18n::money_format(0)?>"
<?=(core::config('advertisement.parent_category')) ? 'data-isparent' : NULL?>
>
<div id="select-category-template" class="col-md-5 hidden">
<select class="disable-chosen select-category" placeholder="<?=__('Pick a category...')?>"></select>
</div>
<div id="paid-category" class="col-md-12 hidden">
<span class="help-block" data-title="<?=__('Category %s is a paid category: %d')?>"><span class="text-warning"></span></span>
</div>
</div>
<?if($id_category !== NULL):?>
<div id="category-edit" class="row">
<div class="col-md-8">
<div class="input-group">
<input class="form-control" type="text" placeholder="<?=$selected_category->name?>" disabled>
<span class="input-group-btn">
<button class="btn btn-default" type="button"><?=__('Select another')?></button>
</span>
</div>
</div>
</div>
<?endif?>
<input id="category-selected" name="category" value="<?=$id_category?>" class="form-control invisible" style="height: 0; padding:0; width:1px; border:0;" required></input>
</div>
</div>