Function php convert string to UTF-8

conver string to Utf8 if String is not equals to 'UTF-8'

/*
*QQ: 290359552
* conver to Utf8 if $str is not equals to 'UTF-8'
*/
function convToUtf8($str)
{
if( mb_detect_encoding($str,"UTF-8, ISO-8859-1, GBK")!="UTF-8" )
{

return  iconv("gbk","utf-8",$str);

}
else
{
return $str;
}

}