Nobody sending bulk messages does so with a GSM modem. For anything beyond a few messages here and there, you should be using an SMS gateway. Otherwise, you are limited to sending one message a time, bandwidth is very limited, and your provider won't like you very much. In fact, bulk sending is usually against the ToS (at least on CDMA providers here in the USA).
If you still want to go with this route, all you need is a way to open a serial port in PHP. Depending on your platform, this is relatively easy to do: http://code.google.com/p/php-serial/
If you need to do this on Windows, I recommend instantiating the .NET Serial class with PHP. See Serial comm with PHP on Windows for alternatives.
From there, all you need to do is send the appropriate commands. These sometimes vary from modem to modem, but you can find the more standard commands here: http://www.smssolutions.net/tutorials/gsm/sendsmsat/
