Brad, for comparison, I'm looking for a PHP script that I can install on my web server that will start my lawn mower for me. It can be stated confidently that no such PHP script exists because there is no connection between the Internet and my lawn mower.
Likewise, SMS messages exist solely within an SMS network (and do not cross modem accessible phone lines) - an SMS Gateway is required. So this isn't a case of not knowing what PHP scripts may or may not exist - it's a case of knowing how the SMS system works.
An SMS service can redirect message from the SMS network to an email address (and email mailboxes can be read over a modem) but the OP is trying to do this on the cheap and avoid a gateway service... so, the answer remains, as originally stated, a simple no, it does not exist.