.ajax() post to callback file does not recognize php define constant - php

Question

When using the code below I am having trouble setting the right location. Somehow in the callback file the php constant is not recognised, thus the rest of the code is not functioning.
snippet from html:
<a href="#" class="selectLocation" id="ABC">
snippet from .js
var locationID;
$(".selectLocation").click(function(){
locationID = this.id;
setLocation();
});
function setLocation() {
$.ajax({
data: {location: locationID},
success: function (data) {
// do something
};
});
};
// DEFAULT AJAX SETUP
$(function () {
$.ajaxSetup({
type: "POST",
url: "dir/callback.php",
dataType: "json",
cache: "false",
error: function (jqXHR, exception) {
if (jqXHR.status === 0) {
console.log('No connecting.\n Verify Network.');
} else if (jqXHR.status == 400) {
console.log('Bad Request. [400]');
} else if (jqXHR.status == 404) {
console.log('Requested page not found. [404]');
} else if (jqXHR.status == 500) {
console.log('Internal Server Error [500].');
} else if (exception === 'parsererror') {
console.log(jqXHR.responseText);
console.log('Requested JSON parse failed.');
} else if (exception === 'timeout') {
console.log('Time out error.');
} else if (exception === 'abort') {
console.log('Ajax request aborted.');
} else {
console.log('Uncaught Error: ' + jqXHR.responseText);
};
}
});
snippet from callback.php:
<?php
header("content-type:application/json");
session_start();
require_once ('config.php');
...
$connect->setLocation($_POST['location']);
the $_POST['location']; will send ABC to the rest of my code instead of the right value defined below.
snippet from config.php:
define('ABC', 'some location');
While if I do:
echo (ABC);
in the callback file, I will get the correct location value.

Answer 1

In your $_POST['location'] variable, there's a string 'ABC'.
So if you do echo($_POST['location']);, you'll get ABC, not some location.
Try this one :
$connect->setLocation(constant($_POST['location']));

Answer 2

If you intend to set the a-element's id property to your ABC-constant, you'd have to do it like this:
<a href="#" class="selectLocation" id="<?php echo ABC; ?>">
PHP constants are only resolved in PHP-code, not in HTML.
Besides that, encoding data in a DOM-element's ID seems somewhat convoluted to me. If I have to encode stuff in HTML elements, I prefer to use hidden span-tags or hidden elements if I'm inside a form. You can also try adding the following to your PHP script producing the HTML file:
<script type="text/javascript">
locationID = <?php echo ABC; ?>;
</script>
Assuming locationID is in global scope. Alternatively, you can provide some kind of init()-function in your JS file and call it via a snippet similar to the above.