I would recommend renaming the function "sms" to "init_sms". This would allow you to provide account details and any other paramters to the library before creating the object. For example:
$mysms = new sms();
$mysms->api_id = "123456";
$mysms->user = "Username";
$mysms->password = "password";
$mysms->use_ssl = true;
$mysms->sending_method = "fopen";
It's easier for maintaining one version of this library and specifying in our own code what parameters we want.
I thought I mentioned this once before with v1.5 or earlier but when I upgraded to v1.6 the new code failed in my scripts.