In order to use your WordPress website’s functions in an external PHP file, you need to load your wp-load.php file accordingly.
Load your wp-load.php file
In your WordPress theme’s folder, create your custom PHP file and add the following line of code at the top:
require_once( '../../../wp-load.php' );
Note, if you’re on version-4 or version-5 (Gutenberg), the above code should work for you.
However, if you’re using a previous version of WordPress, you should add another parent directory path (../, a total of 4-parent paths) into your require_once() function.
Finally, in order to navigate to your file, you should use the direct theme URL as per the example below: