Ajax creating PHP variable from selectbox - php

Question

So i'm trying to make it so when I select something from the first select box will create a PHP variable.
Say in the select box I select option 1, it will create a php variable like $number = 1;. or if I select option 5 it'll create a variable like $number = 5;.
I know this require's ajax, but I have minimal to no experience with this at all.
Code:
<form class="form-horizontal row" id="select-service">
<div class="form-group">
<label class="col-sm-4 control-label">Select Social Media</label>
<div class="col-sm-4">
<select class="form-control" name="category" id="category" onchange="func(this.value)">
<?php
$stmt = $pdo->prepare('SELECT * FROM categories');
$stmt->execute();
if($stmt->rowCount() > 0) {
echo'<option selected="true" style="display:none;">Select a social media</option>';
} else {
echo '<option selected="true" style="display:none;">No social medias are available</option>';
}
while($row = $stmt->fetch(PDO::FETCH_ASSOC)) {
echo '<option value="'.$row['id'].'">'.$row['name'].'</option>';
}
?>
</select>
</div>
</div>
<div class="form-group">
<label class="col-sm-4 control-label">Select Service</label>
<div class="col-sm-4">
<select class="form-control" id="service" onchange="quantity(this.value)">
<option selected="true" style="display:none;">Please select a category.</option>
</select>
</div>
</div>
<ul class="pager wizard">
<div class="pull-right">
<li class="next">
<button href="#" class="btn btn-info"><i>Next</i></button>
</li>
</div>
</ul>

Answer 1

You cannot alter PHP variables on runtime (at least not from the same document you are executing the AJAX script). This is because PHP and jQuery are executed on diferent levels:
PHP is server side processing and jQuery happens on the client side.
Yes. AJAX can be used to make PHP requests but it does not exactly can alter variables in real time.
AJAX is a client side solution for calling external php files. in these files you could do basically anything you can do with a php request:
Read data from server
Insert data to server
Update parameters
Etcetera.
What you could do, depending on what you need is desing your site so what you need could be done using an external php file.
You call myfile.php?number=1 using AJAX
And then in myfile.php
<?php
$number = $_GET["number"]; // this sets the $number var to 1
//process or do whatever you need with the number
echo $number;
?>
This echo call will not be displayed on sceen as it normally is but instead you fetch it to Javascript or jQuery and process whatever you need it for.
I recomend you read the W3 Schools guide on AJAX for JS because it contains examples and it is very well explained on what you can do and how tho acomplish it.