How to set prefix before string generation?

Hi i have a function that generates random string, but i want to set 3 prefix's and system to choose random prefix of thoses specified? The prefix's is 001, 002 or 003. For example when i try to generate string i want my system to choose one prefix of these that i specify.

public function generateRandomNumber($length) {

    $prefix = '001';
    //$prefix = '001';
    //$prefix = '001';
    $characters = '0123456789';
    $charactersLength = strlen($characters);
    $randomNumber = '';
    for ($i = 0; $i < $length; $i++) {
        $randomNumber .= $characters[rand(0, $charactersLength - 1)];
    }
    return $prefix . $randomNumber;
}

via Chebli Mohamed