To cite package 'difR' in publications use:

  D. Magis, S. Beland, F. Tuerlinckx, P. De Boeck (2010). A general
  framework and an R package for the detection of dichotomous
  differential item functioning. Behavior Research Methods, 42,
  847–862.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {A general framework and an R package for the detection of dichotomous differential item functioning},
    author = {D. Magis and S. Beland and F. Tuerlinckx and P. {De Boeck}},
    journal = {Behavior Research Methods},
    year = {2010},
    volume = {42},
    pages = {847--862},
  }