Whats φ symbol in "hf = φ + EK"?

I presume this is in the context of the photoelectric effect, in which case φ is the work function of the metal. This represents the amount of work that needs to be done for the metal to give up an electron, and hence the maximum kinetic energy (EK) that the ejected electrons can have is the energy of the incident photons (hf) minus the work function φ. The work function can also be written as φ = hf0 where f0 is the threshold frequency (and hence energy) of light needed to eject electrons.