In Middle Japanese the [p] sound, which had once disappeared before Old Japanese, appeared again but as it paralleled [ɸ], it was treated as a newly-introduced phoneme/p/, independent from [ɸ] (which is referred to as a phoneme/h/).
中世日本語には上代までに一旦消えた[p]が再び現れたが、[ɸ]と並立することから[ɸ]（音素/h/としておく）とは異なる、新しく導入された音素/p/として扱われる。