show different content to mobile users

I’d like to do something like

if(is_mobile) {
//do something
}else{
//do something else
}

I’ve seen similar for redirecting mobile users to another site, I just want to redirect certain links to different content within the same site, but I’m finding no info, probably not searching with the right terms..
no preference yet as to whether the comdition is for screen size or mobile device, either will do for now.

thanks!