Linking two php dynamic dropdown lists - php

Question

I'm in need of a bit of help >_<. I have two tables:
The first table which populated with car manufacturers called list_vehicle_marks with the columns
id | value
The second table, populated with car models called list_vehicle_models and has the following columns;
id | value | mark_id
The column mark_id has the same value as id in the first table, in other words, Ford has the id no. 37 and all Ford models have the mark_id as 37 too.
How would I create two dynamic dropdown menus, where the second menu shows only the models based on the manufacturer selected in the first menu?
This is what I have done so far:
<?php
connection to host, db, blah blah
if ($db->connect_error) {
echo "Failed to connect to MySQL: (" . $db->connect_error . ") "
. $db->connect_error;
} else {
$sql = "SELECT value FROM list_vehicle_marks";
$result_db = $db->query($sql);
if (!$result_db) {
echo $db->error . ' Error perform query!';
} else {
echo '<select name="value">';
echo '<option value="">Select...</option>';
while ($row = $result_db->fetch_object()) {
echo '<option value="' . $row->value . '">';
echo $row->value;
echo '</option>';
}
echo '</select>';
}
}
$db->close();
?>

Answer 1

In the code you posted, the drop down that you create, it should be something like this
echo '<option value="' . $row->id. '">';
echo $row->value;
echo '</option>';
Notice the $row->id instead of $row->value
To Populate the corresponding dropdown you have two options:
Once a user selects a value from the marks drop down you trigger a PHP function which gets the values of the corresponding vehicle models and re-render the page (You capture the id selected by the user using javascript)
You fetch all the vehicle models along with vehicle marks (add another query to your above code that fetches all the models), keep the result of the models query in a JavaScript variable and generate the second dropdown using javascript once a user makes a selection. Here too you capture the users selection and call a javascript function that spits out a corresponding id drop down.
The second method would not be a good option if your models table has a lot of data.