I'm Sr. Consultant Development, have experience in streaming for IPTV(Roku, Amazon Fire TV, Android TV), mobile(Android & iOS) and web application. I'm also experienced while developing API which is used in Roku channel. Message me if you want to know more about me or my work.
Here are few things which are required to start Roku channel
(1) A video hosting server if you are providing VOD(Video on demand)
(2) A web application that will provide you to upload/ delete video, audio, user, etc(optional)
(3) An Api or web services which is required to communicate between video hosting server and Roku channel
(4) A Roku application that is called Roku channel
As you told in your post, (1) that is hosting server and your video stored at server that means you are able to upload video at server. So you will need only API that return json or xm written in PHP as you told and Roku Channel that will be off-course written in BrightScript. You also need to provide logo and other images withe exact size like header(called Overhange in BrightScript), Splash Screen(optional), etc. I will let you know exact size of graphics
I need to discuss with you after that I will tell you exact time required to build your channel.