Yeah it is possible, and yes you need to have access to an SMS Gateway machine. It's basically a machine configured to receive & send SMS messages to mobile numbers. The other way you could send SMS messages is if you knew the cellular provider for the destination phone number. Most cellular companies allow you to send an SMS message as an email. For example, if the destination phone number is 555-555-5555 and the cell provider is Telus (and you have a simple SMTP server running on your PHP machine), then you could send an email via php to 5555555555@msg.telus.com and the cell phone would receive an SMS containing the contents of your email. Of course you can't send attachments or anything.