WHMCS Integration Problem

I just installed the plug-in with no problem, however when I went to use it after I put in the remote host information, (https://starlitehosting.com/whmcs/) I get this php error:

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

Please advise.

Thanks
s/ Dan