I have tried to make a system that can generate a large random number, and I have encounter with problems. Here is the code that work when you first see it, but then you find there will be a great flaw in the system. I will show you step by step:

function bcrand($min, $max, $rand='rand'){
//ok, first, we subtract $max with $min
//because in the end we are going to
//add the number with $min
$rand_max = bcsub($max,$min);
$max_len = strlen($max);
 
 
 
$rand_num[0] = $rand(0,$max[0]);
  if($rand_num[0] == $max[0]){
    $ismax = 1;
}
 
$i = 1;
do{
  if($ismax){
    $rand_num[$i] = $rand(0,$max[$i]);
    if($rand_num[$i] == $max[$i]){
      $ismax = 1;
    }
  }else{
    $rand_num[$i] = $rand(0,9);
  }
  ++$i;
  usleep(mt_rand(0,10));
}while($i < $max_len);
 
 
//make the $rand_num array into a string
$i = 0;
do{
$return_num .= $rand_num[$i];
++$i;
}while($i<$max_len);
//add the returning number with the min
//number, it also take out the prefix zeros
$return_num = bcadd($return_num,$min);
return $return_num;
}

The concept is pretty easy, generate each digit of the number, and then put them together.
The first problem I meet is there could be a digit larger than the max number I specified. To solve that problem, I made the it generate the first digit equal or less than the max number's first digit first. and if the first digit is the max digit possible, the next digit will follow the same way.
This worked out fine, but if you really think about it, this is not right.
Suppose chose a number between 0 and 1000, if it choses the first digit, there is half of the chance of choosing 1, and lead to half of the chance of getting 1000, and half of the chance get the rest 999 numbers.
So I still have to work on that a bit more.