mihirpatel83,
In order to send SMS via an SMS Gateway, you will first have to find an SMS Gateway.
Some provide PHP API's and can provide specific examples as to how their system works.
Other gateways will allow you to submit a "POST" to their server, as if you have submitted a form from a web page.
Note: Generally - dealing with any kind of professional gateway service you will want to be more experienced in PHP.
The type of website you are dealing with can be sucecptable to attack, especially since it will have access to an SMS Gateway (Spammers would love that access).
If you are totally new to PHP, i'd suggest a less ambitious project before moving onto this.
Also noteworthy - many SMS gateways will offer some assistance in getting their gateway working.