Generates a random decimal with a large number of digits.

big_rand_decimal.php

function big_rand_decimal($integerDigits, $decimalDigits, $decimalSeparator = ".") {
    if(!is_numeric($integerDigits) || $integerDigits <= 0)
        $integerDigits = 0;
    else if(!is_integer($integerDigits))
        $integerDigits = (int)ceil($integerDigits);

    if(!is_numeric($decimalDigits) || $decimalDigits <= 0)
        $decimalDigits = 0;
    else if(!is_integer($decimalDigits))
        $decimalDigits = (int)ceil($decimalDigits);

    if($integerDigits + $decimalDigits == 0)
        return '0' . $decimalSeparator . '0';

    $bigInteger = big_rand_integer($integerDigits + $decimalDigits);
    $bigDecimal = substr($bigInteger, 0, $integerDigits) . $decimalSeparator . substr($bigInteger, $integerDigits);

    return ($integerDigits == 0 ? '0' : '') . $bigDecimal . ($decimalDigits == 0 ? '0' : '');
}

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 decimal. Useful only when the decimal is too complex to be stored within native PHP variables. On extraneous input, it'll return '0.0'; it will always try to give you a valid decimal. If you give it decimal parameters, it'll round them up to the next integers. 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.