PHP - Cómo hacer para que la categoría no sea obligatoria

<?php

 if(isset($_POST['add_product'])){

   $req_fields = array('product-title','product-categorie','product-quantity','buying-price', 'saleing-price' );

   validate_fields($req_fields);

   if(empty($errors)){

     $p_name  = remove_junk($db->escape($_POST['product-title']));

     $p_cat   = remove_junk($db->escape($_POST['product-categorie']));

     $p_qty   = remove_junk($db->escape($_POST['product-quantity']));

     $p_buy   = remove_junk($db->escape($_POST['buying-price']));

     $p_sale  = remove_junk($db->escape($_POST['saleing-price']));

     if (is_null($_POST['product-photo']) || $_POST['product-photo'] === "") {

       $media_id = '0';

     } else {

       $media_id = remove_junk($db->escape($_POST['product-photo']));

     }

     $date    = make_date();

     $query  = "INSERT INTO products (";

     $query .=" name,quantity,buy_price,sale_price,categorie_id,media_id,date";

     $query .=") VALUES (";

     $query .=" '{$p_name}', '{$p_qty}', '{$p_buy}', '{$p_sale}', '{$p_cat}', '{$media_id}', '{$date}'";

     $query .=")";

     $query .=" ON DUPLICATE KEY UPDATE name='{$p_name}'";

     if($db->query($query)){

       $session->msg('s',"Producto agregado exitosamente. ");

       redirect('add_product.php', false);

     } else {

       $session->msg('d',' Lo siento, registro falló.');

       redirect('product.php', false);

     }

   } else{

     $session->msg("d", $errors);

     redirect('add_product.php',false);

   }

 }

?>

<?php include_once('layouts/header.php'); ?>

<div class="row">

  <div class="col-md-12">

    <?php echo display_msg($msg); ?>

  </div>

</div>

  <div class="row">

  <div class="col-md-9">

      <div class="panel panel-default">

        <div class="panel-heading">

          <strong>

            <span class="glyphicon glyphicon-th"></span>

            <span>Agregar Servicio</span>

         </strong>

        </div>

function validate_fields($var){

  global $errors;

  foreach ($var as $field) {

    $val = remove_junk($_POST[$field]);

    if(isset($val) && $val==''){

      $errors = $field ." No puede estar en blanco.";

      return $errors;

    }

  }

}

<input type="hidden" name="product-categorie" value="1">

$p_cat   = remove_junk($db->escape($_POST['product-categorie']));