A coarse potting mix of primarily bark and fibrous peat works best for a Phalanopsis orchid potting mix. It will hold the right amount of moisture to keep the roots moist, but will drain well enough so not to be too wet after watering. A good orchid bark will be slow to decompose when fertilizer is added and will remain good for several years.
If you orchids are regularly too dry, or if you cannot attend to your orchids as often as you like, you can add Sphagnum moss or another fibrous peat to the Phalaenopsis orchid potting mix. This will add slightly more moisture to the root system and will help to maintain the moisture longer. It is best to line the pot or basket with the Sphagnum moss then put in the bark mix and top off with the moss.
Many commercial Phalaenopsis orchid potting mixes also have horticultural foam and charcoal in them. Horticultural foam is a substitute for fibrous peat and is used to hold moisture. Horticultural charcoal is used to help keep the bark from souring and decomposing. Commercial potting mixes are shipped dry so you will need to water the mix you will need for repotting very well and then let drain overnight. The mix will have the right amount of moisture for repotting the next day.
Phalaenopsis orchids are reluctant to grow their roots into a potting mix it does not like and the roots will circle the top edge of the pot or extend over the rim of the post instead of growing down into the mix. If this happens it is most likely that the potting mix needs replaced. Phalaenopsis orchids like loose open mix.
If the Phalaenopsis orchid potting mix smells sour when moist it is probably broken down to the extent that the orchid can no longer benefit from the mix. You may also start seeing small particles of the mix washing through the bottom of the pot or clogging it from draining. If it gets too bad, plants will suddenly loose foliage and shrivel from loosing roots. The sour smell is the first sign leading to root rot. The potting mix should smell pleasant when moist.
Some growers use inorganic materials for Phalaenopsis orchid potting mixes. They use items like Rockwool, Perlite, and Perlag all made from volcanic material. These items last longer, but are difficult to mix correctly, and affect watering schedules. They are not recommended for amateurs.