Long-standing acronyms often leak letters at the end, especially if they've taken on an extraneous look.
For example, MMORPG -> MMO. There's a slight sacrifice of precision ("a Massively Multiplayer Online what?") in return for an acronym that's more morsel than mouthful, and fits easier into Twitter.
I mainly use PH these days. Since it's rarely used in the context of chemical reactions, this usage is quite safe.