Call to undefined function wp_delete_user()

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p.s.: before that line insert:

require_once(ABSPATH.'wp-admin/includes/user.php');

I found this in a Google search and wanted to add this for anyone looking for a solution when using WordPress multisite. Instead of loading wp-admin/includes/user.php, you'll need to load wp-admin/includes/ms.php like:

require_once( ABSPATH . 'wp-admin/includes/ms.php' );