Also been discussed in the following posts:
For a selected shortcode they run about $1000 a month, while a random shortcode might run around $500 a month, in the US.
It is important to notice that 2-Way SMS in the US has strict regulations - you have to use a short code, otherwise your service is considered illegal and the carriers can shut you down. ClickATel does not operate based on shortcodes, they use SMS modems which is basically a way to hook up a cellular line to a computer and intercept SMS messages.
There's quite a bit of details that one should be aware of if one wants to run a business grade SMS service in the US due to the way carriers control the SMS traffic. If there's interest from the readers I'll be more than glad to write down some of the details.
In the US, a Clickatell SMS will come from their shortcode, which is perfectly fine. No need to pay big bucks for your own shortcode, like some SMS providers want you to believe.
If you need to receive SMS, it's a different story. You could get a UK number, which is much cheaper then a US shortcode, but might look weird to your US users.
They have an API.
This only works if you know the carrier that your user uses, because presumably you have to route a reply back to them through an email-to-SMS gateway.
I don't personally know of any frameworks in PHP that will help you, but for Rails there are SMSFu and MMS2R, both of which are good at handling gateway SMS traffic.
You might peek at their code to see how they did it. Both are MIT Licensed.