HI smartsoft,
Are you asking about the login/registration system or just the sms sending from php?
For the sms:
1) Phone Providers Email to SMS gateway
There's quite a few ways you can send sms from a php script, however, all are indirect methods (php cant send sms messages by itself, kinda like saving to a database, you need the db api)
You can use the feature provided by most cell providers that Paradox814 mentioned. Where you just send an email to:
userscellnumber@providerdomain
The provider will receive the email and then forward the email to the user's phone. I think this is the easiest to use.
2) Third Party SMS Gateway
There are also numerous websites that provide an API that allows you to send SMS/txt through their SMS "Gateway" or SMS webservice.
The thrird party sms gateways usually offer different services, like email to sms, or SOAP to SMS, (any web based protocol to SMS) etc.
Since a php server is linked only to the web, and not to an SMS network, 3rd party gateway links the web to the SMS network.
So if you send them an email, they'll forward it to the user via the SMS network. (Some just use the " Phone Providers Email to SMS gateway" which is a bit crappy, but many have their own links the SMS network).
If you make a SOAP call, they'll translate that SOAP request into an SMS message and send it over the SMS gateway.
Same for any other webservice call.
The advantage of the 3rd party gateway is that they offer other features not possible with the Email gateway provided by the Cell Company for users phones.
They most likey offer tracking of the SMS, and stuff like that, and may do your accounting for you etc.
If you want to work with an SMS gateway, what you're actually doing is working with the web protocols, like, Email, SOAP, REST, XML-RPC etc.
They are all socket based protocols, and PHP is good at this. (see php manual for sockets (also called streams), or fsockopen, fopen etc.)
There's also a lot of open source scripts that hide the socket layer from your code and provide an API to interact with web services (eg: nusoap for PHP SOAP clients and servers).
MOst the web services are XML based, there's alot of xml parsers out there for php. (nusoap for example will do the xml parsing for you).
Email is the only txt based one, but that has been abstracted php functions in php like mail() and the IMAP/POP functions.
Most the 3rd party gateways are commercial.
3) Create your own TXT to Web Gateway
You can also purchase hardware to connect your server to the TXT network.
Theres a bunch of equiptment out there for windows that are easy to use and can even be connected to yoru home computer.
These will provide an connection to the txt network, then call a script, like php, that will receive the message.
Saving to the db, and sending your txt over the web etc is up to the script.
good searches are: sms gateway, sms windows gateway, sms linux gateway, sms api (soap api, rest api, email api etc.) etc.
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http://www.mambotastic.com/ is actually an implementation of a third party sms gateway Client as a Mambo/Joomla CMS Component. (You'll need mambo or joomla/installed to install the component and use it)
I cant remember the website that offers the web service right now, its a popular one. What the Component does is connect to this web service for you. plus im too lazy to search...
If you want to work with the web service itself, you can just do a search, or look through the php code for where they'll have something like fsockopen('http://webservice_url.com/etc/etc/')
I believe the component mentions it too...
I'd recommend working with the web service directly. Its more flexible.