随机数：字母

/**
 * 随机数(字母)
 */
function suijizi ($len, $chars=null)
{
    if (is_null($chars)){
        $chars = "abcfghijklmnorstuvwxyzABCDEFGKLMNOPQRSTUVWXYZ012356789";
    } 
    mt_srand(10000000*(double)microtime());
    for ($i = 0, $str = '', $lc = strlen($chars)-1; $i < $len; $i++){
        $str .= $chars[mt_rand(0, $lc)]; 
    }
    return $str;
}