Clickatell is a popular SMS gateway. It works in 200+ countries.
Their API offers a choice of connection options via: HTTP/S, SMPP, SMTP, FTP, XML, SOAP. Any of these options can be used from php.
The HTTP/S method is as simple as this:
http://api.clickatell.com/http/sendmsg?to=NUMBER&msg=Message+Body+Here
The SMTP method consists of sending a plain-text e-mail to: [email protected], with the following body:
user: xxxxx
password: xxxxx
api_id: xxxxx
to: 448311234567
text: Meet me at home
You can also test the gateway (incoming and outgoing) for free from your browser
PHP by itself has no SMS module or functions and doesn't allow you to send SMS.
SMS ( Short Messaging System) is a GSM technology an you need a GSM provider that will provide this service for you and may have an PHP API implementation for it.
Usually people in telecom business use Asterisk to handle calls and sms programming.
You need to subscribe to a SMS gateway. There are thousands of those (try searching with google) and they are usually not free. For example this one has support for PHP.
Source: Stackoverflow.com