<?php
if (isset($son['kat'])) {
    $selectedOption0 = ($son['kat'] == 0) ? "selected" : "";
    $selectedOption1 = ($son['kat'] == 1) ? "selected" : "";
} else {
    $selectedOption0 = "selected";
    $selectedOption1 = "";
}
?>
<select class="form-control-sm" name="kat" id="kat" required>
    <option value="0" <?php echo $selectedOption0; ?>>A Kategori</option>
    <option value="1" <?php echo $selectedOption1; ?>>B Kategori</option>
</select>