How to set up a selection of products by filter?

Question

PHP

Good afternoon!
There is a filter on the site, which should filter by the parameters: "size" and "brand":

The plate in the database, according to which the selection is made:

The selection itself:

//выбираем товар
$st = $pdo->query('SELECT * FROM `brick`');
$oblBricks = $st->fetchAll();

//рез-т получаем в тегах select, option
<form action="" method="post">
                                <p class="lable">размер</p>
                                <select name="obl_size">
                                    <option value="all">все</option>
                                    <?php foreach ($oblBricks as $item): ?>
                                    <option value="<?php echo $item['sz']; ?>"><?php echo $item['sz']; ?></option>
                                    <?php endforeach; ?>
                                </select>
                                <p class="lable">производитель</p>
                                <select name="obl_brand">
                                    <option value="all">все</option>
                                    <?php foreach ($oblBricks as $item): ?>
                                        <option value="<?php echo $item['brand']; ?>"><?php echo $item['brand']; ?></option>
                                    <?php endforeach; ?>
                                </select>
                        <input type="submit" value="go" name="go_filter">
                    </form>

The filter itself is applied next:

if(isset ($_POST['go_filter'])){
    $size = $_POST['obl_size'];
    $brand = $_POST['obl_brand'];
    $stm = $pdo->prepare('SELECT * FROM `brick` WHERE sz=:sz AND brand=:brand ');
    $stm->bindParam(':sz', $size, PDO::PARAM_INT);
    $stm->bindParam(':brand', $brand, PDO::PARAM_INT);
    $stm->execute();
    $Filter = $stm->fetchAll();

//    echo '<pre>';
//    var_dump($Filter);
//    echo '</pre>';
}

The question is what filtering happens if all values ​​are used except for , because there is no "all" or "all" value in the DB. How to make the filter select, for example, by specifying the size - "all", and the manufacturer - "KERMA", that is, specific? Thank you.
<option value="all">все</option>

Answer 1

all, because there is no "all" or "all" value in the DB.

Because and :sz can be all

SELECT * FROM `brick` WHERE ( sz=:sz OR :sz = 'all' ) AND ( brand=:brand OR :brand = 'all' )