How to populate a child drop down menu from parent selection PHP

Question

How would one populate a child drop down selection on a static page from a parent drop down using only PHP. Eg having a child drop down populate with counties after you have selected which state you live in, or in my case populate series after manufacturer.

EDIT: Is there a way to do it without js?

<?php
$man = mysqli_query($con,"SELECT DISTINCT manufacturer FROM inventory.manufacturer WHERE manufacturer!=\"\" ORDER BY manufacturer;");               
echo "<select name=\"manufacturerS\">
<option value=\"\">Select</option>";
while($row = mysqli_fetch_array($man)) {
    echo "<option value=\"".$row['manufacturer']."\">".$row['manufacturer']."</option>";
}   
echo "</td>
<td>";

if(isset($_POST['manufacturerS'])){
$ser = mysqli_query($con,"SELECT DISTINCT series, manufacturer FROM inventory.audit WHERE series!=\"\" AND manufacturer='".$_POST['manufacturerS']."' ORDER BY series;");               
echo "<select name=\"seriesS\">
<option value=\"\">Select</option>";
while($row = mysqli_fetch_array($ser)) {
    echo "<option value=\"".$row['series']."\">".$row['series']."</option>";
}   
    echo "</td>
    <td>";      
}

Answer 1

You need of client-side to determine when manufacturerS select was selected, then you can use pure javascript :

<form id="manufacturerF" method="POST">
<select name="manufacturerS" onchange="document.getElementById('manufacturerF').submit();">
<?php  
    $man = mysqli_query($con, "SELECT DISTINCT manufacturer 
    FROM inventory.manufacturer 
    WHERE manufacturer!=\"\" 
    ORDER BY manufacturer;");

    while ($row = mysqli_fetch_array($man)) {
        echo '<option value="'.$row['manufacturer'].'>'.$row['manufacturer'].'</option>';
    } 
?>
</select>
</form>


<?php
    if(isset($_POST['manufacturerS']) && !empty($_POST['manufacturerS'])){  
        echo '<select name="seriesS">';

        $ser = mysqli_query($con,"SELECT DISTINCT series, manufacturer 
        FROM inventory.audit 
        WHERE series!=\"\" 
        AND manufacturer='".$_POST['manufacturerS']."' 
        ORDER BY series;");

        while ($row = mysqli_fetch_array($ser)) {
            echo '<option value="'.$row['series'].'">'.$row['series'].'</option>';
        }   

        echo '</select>';   
    }
?>

I recommend you use JQuery so will not need a <form> and reload your page. The correct idea would be to use JQUERY to detect when the select1 was selected and then with AJAX populate the select2