Generates a random integer with a large number of digits.

big_rand_integer.php

function big_rand_integer($digits) {
    if(!is_numeric($digits) || $digits <= 0)
        return '0';

    if(!is_integer($digits))
        $digits = (int)ceil($digits);

    $multiplier = (int)ceil($digits / 9.0);

    $function = function() {
        return(str_pad(strrev((string)mt_rand()), 9, '0', STR_PAD_LEFT));
    };

    return substr(str_repeat_func($function, $multiplier), 0, $digits);
}

function str_repeat_func($function, $multiplier) {
    if(!is_callable($function) || !is_integer($multiplier) || $multiplier <= 0)
        return "";

    $accumulator = "";
    for($i = 0; $i < $multiplier; $i++)
        $accumulator .= call_user_func($function);

    return $accumulator;
}

Returns a string with the number. Useful only when $digits > 9. On extraneous input, it'll return '0'. If you give it a decimal, it'll round it up to the next integer. It uses the mt_rand function internally. It attempts to save cycles by using the 9 useful digits of mt_rand(mt_get_max_rand) instead of calling it once per digit.