Detect mobile and redirect

I need some help finding a script (php?) to detect visitors who are connecting to my site on mobile phones or other small screen devices, and then redirects them to a subdomain optimized for small screens. Can someone point me to a “simple” script to do that? I was not able to find a thread on this forum that explains to a non-php/js programmer how to do this.

Also in #Full-stackDeveloper _↴