Phal in need of assistance

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Don't cut any root that appears to be alive. The big one on the left and the good sized one on the right look alive to me. The brown ones in the middle may be dried up and hollow. A good test is if you wet them and they turn green they are alive. Some live roots can be brownish too tho. If they are hollow or mushy them trim them off. The plant looks dehydrated. The roots it has should sustain it until the tiny new ones get bigger. I like a mix of medium bark and moss for my Phals, but everyone has their own growing conditions and different things work for different people You mention you have some Phals. If they are doing well, then just pot this one the same. Be sure to give it good aeration, a smallish pot that fits the roots and allow it to come close to dry before watering again. Some KLN or seaweed would help speed up root growth if you have it.