WHMCS Integration Plugin Doesn't Work!

I just bought the WHMCS WP Integration plugin from yourself, installed it into my Wordpress site (http://z01.co.uk) and when I attempt to link it to my WHMCS installation (http://oracle.z01.co.uk) by entering its location into Remote WHMCS Host text area of the plugin's control panel, I get the following error message on clicking Save.

I get this if I enter the address as http://oracle.z01.co.uk

Fatal error: Call to undefined function mb_strrchr() in /home/z01/public_html/wp-content/plugins/whmcs-wordpresss-integration/lib/url_to_absolute.php on line 119

If I enter the address as oracle.z01.co.uk, I just get the settings saved message however if I visit http://z01.co.uk/my-account/ which includes your integration shortcode I get the following error on page:

WHMCS Integration: A valid URL was not provided.

Also it would appear that the address hasn't been saved to the plugins control panel.

If this plugin isn't going to work I want a refund or a fix immediately.

Thanks

JB