Fatal error: Call to undefined function mb_internal_encoding() – functions.php

I have installed the latest version of WP 3.4 and everything is working fine. But when installing some themes, (for eg: any of the four themes in the URL: http://www.free-wordpress-theme.net/theme/commercial-construction-theme-two.html), I get the following error:

Fatal error: Call to undefined function mb_internal_encoding() in ../blog/wp-content/themes/home_repair_wp4/functions.php on line 12

Is there any fix for this so I can get these theme working.