Chose from multiple array(s) with bias

I am looking for a simple/easy way to get a script to pick one from a selection of different array's then pick values within them. i need each one to have its own probability.

Currently i have it setup like

    $item = array
    (
   array("item1"),
   array("item2"),
   array("item3,item4,item5"),
   array("item6,item7"),
   array("item8,item9,item10"),
    );
   function getrandomelement($Void) {
   $pos=rand(0,sizeof($Void)-1);
   $res=$Void[$pos];
   if (is_array($res)) return getrandomelement($res);
   else return $res;
   }
   echo getrandomelement($Void);
   ?>

..

while this is working for a random pick..it does not have the "rarity" i need, so i need a way for it to pick from the following.

    $Commonitem = Array(
    (
    array("item1"),
    array("item2"),
    );

    $rareitem = Array(
    (
    array("item3","item4","item5"),
    array("item6"),
    );
    $rareitem2 = Array(
    (
    array("item7","item8","item9"),
    array("item10"),
    );

so it need to roll the array to pick, then the "item" based on a set bias.