<form>
    <select name="cat[1]">
        <option value="food">FOOD</option>
        <option value ="electronics">ELECTRONICS</option>
    </select>
    <select name="subcat[1][]" multiple>
        // get this from ajax
        <option value="1">Bakes & Nuts</option>
        <option value="2">Drinks</option>
    </select>
    <input type="file" name="catimage[2]">

    <select name="cat[2]"> // assign key with javascript
        <option value="food">FOOD</option>
        <option value ="electronics">ELECTRONICS</option>
    </select>
    <select name="subcat[2][]" multiple> // assign key with javascript
        // get this from ajax
        <option value="1">Bakes & Nuts</option>
        <option value="2">Drinks</option>
    </select>
    <input type="file" name="catimage[2]"> // assign key with javascript

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

<?php
foreach ($_POST['cat'] as $key => $value) {
    // sanatize/validate your inputs
    $cat = $value;
    $subcat = implode(',', $_POST['subcat'][$key]);
    $file = $_FILES['catimage'][$key]
    // insert
}
?>