Связанные списки ajax+php

<?
define('DB_HOST', 'localhost');
define('DB_USER', 'root');
define('DB_PASS', '');
define('DB_NAME', 'mvc');
$pdo = new PDO('mysql:host='.DB_HOST.';dbname='.DB_NAME.';charset=utf8', DB_USER, DB_PASS);
$res = $pdo->query('SELECT DISTINCT themes FROM books WHERE themes IS NOT NULL');
$themes = $res->fetchAll(PDO::FETCH_COLUMN);
?>
 
<form id="form-books">
    <select name="themes" id="themes">
        <?
        foreach($themes as $theme){
            ?>
            <option value="<?=$theme?>"><?=$theme?></option>
            <?
        }
        ?>
    </select>
    <select name="category" id="category"></select>
    <select name="book" id="book"></select>
</form>
 
 
<script>
    const themes = document.getElementById('themes');
    const category = document.getElementById('category');
    const book = document.getElementById('book');
 
    themes.addEventListener('change', function(e){
        let selectTheme = themes.options[themes.selectedIndex].value;
        let req = new XMLHttpRequest();
        req.addEventListener('readystatechange', function(){
            if(req.readyState == 4){
                if(req.status == 200){
                    let response = JSON.parse(req.responseText);
                    // let str = '';
                    // for(let i = 0; i< response.length; i++){
                    //     str+=`<option value="${response[i]}">${response[i]}</option>`;
                    // }
                    let str = response.reduce(function(s, current){
                        return s+`<option value="${current}">${current}</option>`;
                    },'');
                    category.innerHTML = str;
                }
            }
        });
        req.open('POST', 'functions.php', true);
        req.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
        req.send('theme='+selectTheme+ '&func=getCategories');
    });
    
</script>

<?
define('DB_HOST', 'localhost');
define('DB_USER', 'root');
define('DB_PASS', '');
define('DB_NAME', 'mvc');
$pdo = new PDO('mysql:host='.DB_HOST.';dbname='.DB_NAME.';charset=utf8', DB_USER, DB_PASS);
$func = isset($_POST['func'])?$_POST['func']:null;
if($func){
    $func();
}
function getCategories(){
    global $pdo;
    $theme = $_POST['theme'];
    $pz = $pdo->prepare('SELECT DISTINCT category FROM books WHERE category IS NOT NULL AND themes=?');
    $pz->execute([$theme]);
    $categories = $pz->fetchAll(PDO::FETCH_COLUMN);
    echo json_encode($categories);
}
 
?>