Basic Usage:

require_once 'clickatell.php';

# initialize class
$sms = new Clickatell();

# Prepare and send
$sms->to  = "002154584";
$sms->msg = "This is a text";
$sms->send();

in the example above I assume that you have added your clickatell username, password and api_id in the class file
if not then you would do something like this:

require_once 'clickatell.php';

# initialize class
$sms = new Clickatell();

# set clickatell credentials
$sms->username = "myuser";
$sms->password = "mypass";
$sms->api_id   = "myapiid";

# Prepare and send
$sms->to  = "002154584";
$sms->msg = "This is a text";
$sms->send();

The class Send function will either return a message ID (if send succeed) or return false if failed.
if you are developing and want to debug then simply add this before using $sms->Send()

$sms->debug = true;

then the Send function will either return a message ID on success or return something like this:

Type: Error
Error Code: 135
Error Message: Authentication Failed

You can download Clickatell PHP Class from bitbucket
https://bitbucket.org/amaroks/clickatell_php/src

You can see the Facebook sms app I built here:
http://apps.facebook.com/sms_world/

Let me know if you have any questions!