Untitled

<?php
if (isset($_POST['submit'])) {
    foreach ($_POST['cat'] as $key => $value) {
            echo $cat = $value . '<br>';
            echo $subcat = implode(',', $_POST['subcat'][$key]) . '<br>';
    }
}

?>

<!DOCTYPE html>
<html>
<body>
<form action="" method="POST">
    <select name="cat[1]">
        <option value="food">FOOD</option>
        <option value ="electronics">ELECTRONICS</option>
    </select>
    <select name="subcat[1][]" multiple>
        <option value="1">Bakes & Nuts</option>
        <option value="2">Drinks</option>
    </select>

    <select name="cat[2]"> // assign key with javascript
        <option value="food">FOOD</option>
        <option value ="electronics">ELECTRONICS</option>
    </select>
    <select name="subcat[2][]" multiple>
        <option value="1">Bakes & Nuts</option>
        <option value="2">Drinks</option>
    </select>

    <input type="submit" name="submit" value="submit">
</form>
</body>
</html>