PHP can't find MySQL support but Wikka requires MySQL. Please check the output of phpinfo() in a php document for MySQL support: it needs to be compiled into PHP, the module itself needs to be present in the expected location, and php.ini needs to have it enabled.
Also note that you cannot have mysqli and mysql support both enabled at the same time.
Please double-check all of these things, restart your webserver after any fixes, and then try again!