There exists other gateways as well. http://www.aspsms.com/ is the one
I use to send SMS with PHP. Some guy wrote a nice PHP class that
handles everything between the gateway and your script:
http://www.net-track.ch/opensource/phpaspsms/

Then, sending an SMS becomes as easy as:

$sms->setOriginator("...");
$sms->addRecipient("...");
$sms->setContent("An SMS message.");

Usage of al those SMS gateways is not free though, but prices are
cheaper than sending SMS with mobile phones.