You should move this code into a new plugin instead, e.g. save this file as wp-content/plugins/example-ajax-handler.php
<?php
/*
Plugin Name: Example AJAX handler
Description: Handle example_ajax_request admin-ajax requests
Version: 0.0.1
*/
add_action( 'wp_ajax_example_ajax_request', 'example_ajax_request' );
add_action( 'wp_ajax_nopriv_example_ajax_request', 'example_ajax_request' );
function example_ajax_request() {
if ( isset( $_REQUEST ) ) {
$postID = $_REQUEST['postID'];
print $postID;
}
wp_die();
}
then you’ll need to activate it from your admin site’s plugin menu. Then it should work! See the Plugin Basics documentation.