value of drop down 2 depends on value of drop down 1 ; PHP

I have two drop down menus that are being dynamically populated from an mysql database.
Drop down 1 will look like this:

$stmt2 = $db->query('SELECT vendor_id, vendor_name from vendor_inbound ');
$vSelect2 = '<select size="1" name="vendor">' . "\n";
while($row2 = $stmt2->fetch(PDO::FETCH_ASSOC)){
$vSelect2 .= '<option value="'.$row2['vendor_id'].'">'.$row2['vendor_name'].'</option>' . "\n";
}//end while
$vSelect2 .= '</select>' . "\n";

Select allOpen in new window

.

Now depending on the vendor ID my next field on my form (which is a drop down menu) needs to be populated accordingly. My table looks like this;

ph_id      vendor_id              number      status
1                 1                        616      1
2                 1                        214      1
3                 2                        845      1
4                 2                        801      1

My second menu item should look like this:

$stmt3 = $db->query('SELECT number from did where vendor_id ="$id" ');
$vSelect3 = '<select size="1" name="did">' . "\n";
while($row3 = $stmt3->fetch(PDO::FETCH_ASSOC)){
$vSelect3 .= '<option value="'.$row3['number'].'">'.$row3['number'].'</option>' . "\n";
}//end while
$vSelect2 .= '</select>' . "\n";

Select allOpen in new window