Problem coding AJAX in WordPress Plugin

Hi guy’s, banging my head against a wall here trying to get AJAX to work in a wordpress plugin that I am writing. Any help would be massively appreciated!

Thanks in advance,
Sean

Here is where I am at –

Below is an excerpt from my main plugin file where I register the JavaScript file I use to make the ajax call and set up my ajax handle. My understanding is the wp_localize_script should pull the PHP file I use to process the ajax request in to the wordpress fold and allow it access all the WordPress functions. This is where I think my issue lies.

add_action( 'wp_enqueue_scripts', 'so_enqueue_scripts' );

function so_enqueue_scripts(){
   wp_register_script( 'ajaxHandle', plugins_url() . '/custom-registration/js/call_ajax.js', array('jquery'), false, true );
   wp_enqueue_script( 'ajaxHandle' );
   // Localise the PHP script that will handle the AJAX request
   wp_localize_script( 'ajaxHandle', 'ajax_object', array( 'ajaxurl' => plugins_url( 'custom-registration/process-ajax.php' ) ) );
 }

Below is the JS file that makes the call –
/custom-registration/js/call_ajax.js

jQuery(document).ready( function($){
	$('form.billing-form').submit(function(event) {

	event.preventDefault();
	var data = {
	    action:  'my_ajax_action',
	    name:    'my name',
	    message: 'my message'
	}

	$.ajax({
		url: ajax_object.ajaxurl,
		type:"POST",
		data: data,
		success: function( data ) {
			alert( 'AJAX Success');
		},
		error: function( ) {
			alert( 'AJAX failed');
		}
	})
});

Here is my PHP file that should process the request –
/custom-registration/process-ajax.php

add_action('wp_ajax_my_ajax_action', 'my_ajax_action_callback');
function my_ajax_action_callback() {
    echo 'It works!';
    wp_die();
}

The PHP file gets hit but a fatal error is thrown because –

PHP LOG

Call to undefined function add_action()

add_action() is a wordpress function and my process-ajax.php mustn’t be wired up correctly to access the WP functionality.