creating a group of random numbers that is unique

It all depends on what you mean by "unique" and what you will do with those numbers. In principle, uniqid () returns values in hex that you can convert to decimals:

$uid = hexdec(uniqid());

Another solution would be this:

$digitos = '0123456789';
$tamanho = 16; // por exemplo...
$uid = '';
for ($i = 0; $i < $tamanho; $i++) {
    $uid .= $digitos[rand(0, strlen($digitos) - 1)];
}

(Solution based in this code )

The result would be a random numeric string with 16 digits, for example.

You can use microtime

  

microtime - Returns a Unix timestamp with microseconds

Depending on the purpose can be a simple and viable alternative and with low risk of collision.

echo microtime();
0.97959400 1407273578

You can remove the. and space

Just go including the numbers in an array, checking first if it does not already exist in it.

$sorteados = array();

for($i = 0; $i <= 1000; ++$i) {
    do {
       $nr = rand(1000,10000);
    } while (in_array($nr,$sorteados));
    $sorteados[] = $nr;
}

print_r($sorteados);