Sorry! I should have looked at both questions! If it is the flowering stem that is turning brown, that is normal once all of the flowers have faded, though many Phalaenopsis will produce flowering branches off of the main stem, if they are happy. If the base of the plant is turning dark brown and mushy, it may have had too much water, or not enough drainage, and is rotting. Unfortunately, once this starts, it is usually too late to save the plant...hope that's not it!

Oh yes, I should have looked at both questions too.
I agree here with what Tugbrethil says, but I will repeat that as soon as flowering is over, a phalaenopsis stem should be cut off at the base. :)