Bad Code for generating a "random" password

AuthUtility.php

    /**
     * Generate a random alphanumeric password
     * precondition: len must be <= 32
     * param len: length of the random password
     * param enforce_rules: enforce password security rules
     * returns: the random password
     **/
    public static function generateRandomPassword($len=8, $enforce_rules = false)
    {
        $count = 0;
        $valid = true;
        do {
            $password = substr(md5(microtime()), 0, $len);
            if ($enforce_rules)
            {
                try {
                    AuthUtility::checkPassword($password);
                    $valid = true;
                }
                catch (Exception $e) {
                    $valid = false;
                }
            }
        } while ($valid === false && ++$count < 10);

        if ($count >= 10)
        {
            Log::writeln("Unable to generate random password!");
            $password = "6f33b5a3";
        }

        return $password;
    }