How to make it select <script> function work? - php

Question

I have a problem about select function. I have two file, one is frontend, one is backend. How I can make the select function to link with backend file?
Below is my coding:
Frontend file (user_list.php) ?f is represent backend file -
<script>
$("#merchant_type").click(function(){
$.ajax({
type: 'POST',
url: "?f=user_list",
if($("#merchant_type option:selected").text() == "Please Select"){
$("#merchant_id").val('Please Select');
}
});
});
</script>
Backend file (user_list.php)
<label for="cp8" class="control-label col-lg-4">Merchant</label>
<div class="col-lg-5">
<select class="form-control required" id="merchant_id" name="merchant_id">
<option value="" selected>Please Select</option>
<!--<option value="0" selected>All</option>-->
<?php
$sql_branch = 'SELECT * FROM merchant_list;';
$arr_branch = db_conn_select($sql_branch);
foreach ($arr_branch as $rs_branch) {
echo '<option value="' . $rs_branch['id'] . '">' . $rs_branch['shop_name'] . '</option>';
}
?>
</select>
</div><br><br><br>
<label for="cp8" class="control-label col-lg-4">Merchant Types</label>
<div class="col-lg-5">
<select class="form-control required" id="merchant_type" name="merchant_type" disabled>
<option value="0" selected>Please Select</option>
<option value="1">Event Sponsored</option>
<option value="2">Normal Merchant</option>
</select>
</div><br><br><br>
My output select option is below:
For example:
First I choose the Merchant option is "PALVO", then I choose the Merchant Types is "Event Sponsored". After that, if I want go back the Merchant to change other option, the Merchant Types will automatically turn to "Please Select" option. Now the select function can't work because can't link to the backend file.

Answer 1

On change merchant,merchant type will change to default
$(document).on("change","#merchant_id",function(){
$("#merchant_type").val('0');
})
Here Default value for merchant type is 0