Philes commonly refer to files about hacking in the days of BBS

Bulletin board system

A Bulletin Board System, or BBS, is a computer system running software that allows users to connect and log in to the system using a terminal program. Once logged in, a user can perform functions such as uploading and downloading software and data, reading news and bulletins, and exchanging...

s. Ph may have been originally used as homage to the phone/phreak.

Other common abstractions of the word are t-files (shorthand for textfiles) and g-files (general files (etymology not certain)). The use of ph in lieu of f is an homage to the word phone.

Their contents could vary from actual hacking

Hacker (computer security)

In computer security and everyday language, a hacker is someone who breaks into computers and computer networks. Hackers may be motivated by a multitude of reasons, including profit, protest, or because of the challenge...

 and phone phreaking, sexual content

Sex

In biology, sex is a process of combining and mixing genetic traits, often resulting in the specialization of organisms into a male or female variety . Sexual reproduction involves combining specialized cells to form offspring that inherit traits from both parents...

, humour

Humour

Humour or humor is the tendency of particular cognitive experiences to provoke laughter and provide amusement...

, technical information, down to subversive or so-called anarchy material about controversial topics such as poison brewing

Poison

In the context of biology, poisons are substances that can cause disturbances to organisms, usually by chemical reaction or other activity on the molecular scale, when a sufficient quantity is absorbed by an organism....

, homemade explosives, suggestions for making harmful pranks, urban terrorism

Terrorism

Terrorism is the systematic use of terror, especially as a means of coercion. In the international community, however, terrorism has no universally agreed, legally binding, criminal law definition...

 "tactics", building homemade weapons and other similar information of dubious reliability, e.g. instructions for making a nuclear reactor

Nuclear reactor

A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...

out of a plastic barrel to keep in one's garden.