OK, if your mobile is attached to the COM port (i.e. rs232 serial port - *NOT* usb), then you should download the php_serial class.
Then look at this blog for some notes on how to send AT codes. The correct AT codes to use are in some of the previous links I posted.

Hi friend...I have been into this SMS gate way stuff ..I used many sms gateways like OZEKI ,Mcore etc. ...But the problem is all of them are paid softwares and so it does have limitations.So i tried making an sms gateway of my own and by gods grace i succeded.I used Serial port communication using c#.net and AT commands for communicating with the modem. ..Its working perfectly for me.I can send and receive sms using this ... I put some tutorials in my website
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I know this tutorial is not complete,but it covers the most tedious task i faced,ie to convert text message to PDU format which most mobile phones recognizes. The rest is not that tough

I can get you the code if you need.Please contact me ...
Or else if u prefer using OZeki ,here is the tutorial for using those kind of stuffs
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Thanx