Change the value of second select on first select - php

Question

I'm sorry I haven't included "my attempt" as such with this one, I'm useless with jquery so need some advice!!
I would like to change the value of a second selctor based on the results of the first.
I have a database of builders and regions with the headers builder_name and builder_region. The list ends up looking like this ...
builder_1 region_1
builder_1 region_2
builder_2 region_1
builder_3 region_1
builder_3 region_2
builder_3 region_3
(You get the idea)
I'm using the following in the form I've built to get a list of the builders for the first select box ...
echo '<select class= "ml-select" name="gen_builder">';
echo '<option value=" ">Select Builder</option>';
while($row = mysql_fetch_array($rsBUILDER)) {
if($linebreak !== $row['builder_name']) {
echo '<option value="'.$row['builder_name'].'">'.$row['builder_name'].'</option>';
}
else {echo "";}
$linebreak = $row['builder_name'];
}
echo '</select>';
The $linebreak is to get rid of the duplicate builder names from the list, which works a treat.
What I would like to achieve is a second selector that selects the regions available, dependant upon the builder that has been selected in the first option. I hope this makes sense????
The second query needs to look at the builder selected in the first selector and filter out just the regions that are in rows with the builder name form selector one.
Please do say if you need further information, I'm not great at explaining myself.

Answer 1

As you said you don't have experience with jQuery or Ajax, I'll post my answer with as many comments as possible. I will assume that you have two select dropdowns in your page.
The first one contains the builders, so it will have id="builders".
The second one contains the regions, so it will have id="regions".
From what I understand, the first select will be exactly the one you posted in your question, generated server-side (by PHP). I only ask that you please make a slight change on it, making each option value be equal to the builder's database ID, and not its name (unless the builder's primary key is their name, and not an ID). This will make no difference for the final user but will be important for our jQuery solution. The second one will be empty, as the idea is to fill it dynamically with the regions related to the builder selected in the first dropdown.
Now let's get to the jQuery code:
//Everything that goes below this first line will be ready as soon as the page is fully loaded
$(document).ready(function() {
//The following code defines an event. More precisely, we are defining that the code inside it will run every time our select with id "builders" has its value changed
$('#builders').change(function() {
//$(this) is our builders select. $(this).val() contains the selected value, which is the ID of our selected builder
var currentValue = $(this).val();
//Now, this is our Ajax command that will invoke a script called get_regions.php, which will receive the builder's ID in $_GET['builder_id'] and you can use to query your database looking for all regions related to this builder. Make sure to create an array with the returned regions. Your get_regions.php's last line should be echo json_encode($regions);
$.get("get_regions.php", {'builder_id': currentValue}, function(data) {
//Inside this function is the code that will run when we receive the response from our PHP script. It contains a JSON encoded list of regions, so first of all we need to parse this JSON
var regions = $.parseJSON(data);
//Before filling our second select dropdown with the regions, let's remove all options it already contains, if any
$('#regions').empty();
//Now, all there is left is to loop over the regions, adding each as an option of the regions dropdown. I'll do it the universal way
for (var i = 0; i < regions.length; i++) {
var regionOption = '<option value="'+regions[i]['region_name']+'">';
regionOption += regions[i]['region_name'];
regionOption += '</option>';
$('#regions').append(regionOption);
}
});
});
});
Despite any syntax errors (can't test the code from here) this should do the trick. Hope the comments were clear enough for you to understand how things work in jQuery.