How to create a random password in PHP using regex?

Before marking the answer as duplicate, I tried the answer given here and it works. However I want to try with one of the answers given there using openssl_random_pseudo_bytes. This is the code I am using

 public static function GenerateRandomPassword($length)
{
    $randomBytes = self::GetRandomBytes ($length + 1);
    return substr(preg_replace("/^(?=.*[A-Za-z])(?=.*\d)(?=.*[$@$!%*#?&])[A-Za-z\d$@$!%*#?&]/", "", base64_encode($randomBytes)),0,$length);
}

public static function GetRandomBytes($nbBytes = 32)
{
    $bytes = openssl_random_pseudo_bytes($nbBytes, $strong);
    if (false !== $bytes && true === $strong)
    {
        return $bytes;
    }
}  

What I am trying to do is create a password that is 8 alphanumeric characters with at least 1 number and 1 special character. I know I am doing something wrong in the code but couldn't figure out what is wrong. How to modify this function to successfully generate a password with my requirements? Any idea? If this is not possible, I would switch to using the accepted answers given in the other question.