Wordpress：如何使用get_categories

I have a ajax call to a php file which uses get_categories(). But it always says "Call to undefined function get_categories()" How can I make it to work? What should be included?

<?php
if(isset($_GET['slug'])) {
    echo get_categories(array('taxonomy' => 'product_cat'))[0]['slur'];
}
?>

In your ajax php file put below line in the top.

include_once('../../../wp-load.php');