Add function result to database

I am using the following code to generate random numbers. With the probability of getting lower numbers higher than getting high numbers.

function fun() {
        $num = mt_rand(1, 100);
        if ($num > 0 && $num <= 90) {
            $return = mt_rand(10, 45) / 10;  // 90%
        } else if ($num > 90 && $num <= 98) {
            $return = mt_rand(45, 100) / 10;  // 8%
        } else {
            $return = mt_rand(100, 200) / 10;  // 2%
        }
        return sprintf("%01.1f",$return);
    }

However my coding experience is very basic, I am essentially teaching myself and I am unsure as to how to get the result of this function into a form that I could then add to my database..

I know the below won't make sense but maybe it will make my goal clearer..

$interest = (the result of fun()) 
mysql_query("UPDATE r$game[0]_$tab[pimp] SET suisseint=$interest WHERE id=$user1no[id];"); 

Thank you!