Creating unique array keys

I wrote the function below in a recent project, I needed to create unique keys for a small multidimensional array and store it in a session.

<?php
function createUniqueId($length = 6, array $db)
{
    $alph = array("a", "b","c","d","f","g","h","j","k","l","m","n","p","r","s","t","v","w","x","y","z");
    $nums = array(1,2,3,4,5,6,7,8,9);

    $id = "";
    srand((double)microtime()*1000000);
    $max = $length/2;

    for($i = 1; $i     {
        $id .= $alph[rand(0,20)];
        $id .= $nums[rand(0,8)];
    }

    // check if the key already exists
    if (!array_key_exists($id, $db)) {
        return $id;
    }
    else {
        // if it does,lets recurse the function
        createUniqueId($length, $db);
    }
}
?>