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: sms@messaging.clickatell.com, 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