Populate Select based on another Select - php

Question

I'm trying to populate a Select Box based in the choice of the first Select Box, but I'm having some troubles doing it. Simply nothing happens.
What I have until now (this is on my footer):
<script src="./plugins/bower_components/jquery/dist/jquery.min.js"></script>
<script type="text/javascript">
$(document).ready(function(){
$("select.category").change(function(){
var selectedCategory = $(".category option:selected").val();
$.ajax({
type: "POST",
url: "../inc/get_subcat.php",
data: { category : selectedCategory }
}).done(function(data){
$("#subcategory").html(data);
});
});
});
</script>
The page with the div is located at /pages/file.php:
<div class="col-md-6">
<div class="form-group">
<label class="control-label">SubCategory</label>
<div id="subcategory"> </div>
</div> </div>
The file that does the query is in another folder (inc/get_subcat.php):
<?php
include("config.php");
if(isset($_POST['category'])){
$cat = validar($_POST['category']);
$query = "SELECT id, subcategory FROM subcategories WHERE catid = ?";
#Prepare stmt or reports errors
($stmt = $mysqli->prepare($query)) or trigger_error($mysqli->error, E_USER_ERROR);
#Execute stmt or reports errors
$stmt->bind_param("i", $cat);
$stmt->execute() or trigger_error($stmt->error, E_USER_ERROR);
#Save data or reports errors
($stmt_result = $stmt->get_result()) or trigger_error($stmt->error, E_USER_ERROR);
#Check if are rows in query
if ($stmt_result->num_rows>0) {
echo "<select class='form-control' id='subcategory' name='subcategory'>";
# Save in $row_data[] all columns of query
while($row_data = $stmt_result->fetch_assoc()) {
# Action to do
echo "<option value='$row_data[id]'>$row_data[category]</option>";
}
echo "</select>";
}
$stmt->close();
}
?>
There is someone that could help me trying to fix this error? Can't fix despite trying for some hours.
Thanks!
Edit:
This is the HTML code with all Select Box:
<div class="row">
<div class="col-md-6">
<div class="form-group">
<label class="control-label">Category</label>
<select class='form-control' id='category' name='category'><option value='1'>Categoria 1</option><option value='2'>Categoria 2</option></select> </div>
</div>
<div class="col-md-6" id="subcategory">
</div>
</div>

Answer 1

In your HTML you have<select class='form-control'...etc
So the <select> has a class "form-control" only.
But your selector is $("select.category").change...etc. which means it's looking for a select with "category" in the class attribute (the dot . in the selector signifies a class). Such an element doesn't exist, hence why nothing happens.
Perhaps you meant to use the ID (in which case $("#category").change...etc)? Either that or you need to write <select class='form-control category'...etc before it will work.

Answer 2

May be problem in your array data or query. So please check it first.
I post here working example of depending drop down may be this will help you.
PHP code
$sub_category // Your array of Subcategory;
if ($sub_category) {
echo '<option value="">Sub Category</option>';
foreach ($sub_category as $sub_category) {
$selected = '';
if($sub_category['sub_category_id'] == $data['product']['sub_category_id'])
{
$selected = 'selected';
}
echo '<option value="' . $sub_category['sub_category_id'] . '" '.$selected.'>' . $sub_category['sub_category'] . '</option>';
}
}
JS code
<script type="text/javascript">
loadSubCategory();
jQuery(document).ready(function($) {
$(document).on('change', '.category_id', function(event) {
event.preventDefault();
loadSubCategory();
});
});
function loadSubCategory()
{
var category_id = $(".category_id option:selected").val();
$.ajax({
url: 'Your Path Of server Function',
type: 'post',
data: {category_id: category_id, product_id : "<?= $product['product_id']?>" },
cache: false,
success: function (resp) {
if(resp)
{
$('.sub_category_id').html(resp);
} else {
$('.sub_category_id').html('<option value="">Sub Category</option>');
}
}
})
}
</script>
HTML Code
<div class="form-group">
<b>Product Category </b>
<select class="form-control category_id" id="category_id" name="category_id">
<option>Category</option>
<?php
<option value="<?= $value['category_id'] ?>" <?=$selected ?> ><?= $value['category'] ?></option>
</select>
</div>
<!-- Category End -->
<!-- Sub Category Start -->
<div class="form-group">
<b>Product Sub Category </b>
<select class="form-control sub_category_id" name="sub_category_id">
<option value="">Sub Category</option>
<option value="">Sub Category</option>
<option value="20">Books Stationery</option>
<option value="21">Gaming and Accessories</option>
<option value="22">Music Musical Instruments</option>
</select>
</div>