Combining a mobile application with your php website to send automated sms? [closed]

I’ve been learning about web development and one of my colleagues asked me if I could send sms from my website upon user registration. I thought of APIs but those are expensive and for business users and I want something that I can use for myself, something I could use personally and work on. I thought of an idea that maybe it’d be possible to first build an android app and then connect it to my website and whenever a user registers, it could directly send the message to my app and then the app could send the message as sms from the mobile phone. Is it possible, if yes. Could someone guide me to the right direction?