To cite the 'PP' package in publications use:

  Reif, M. and Steinfeld, J. (2021). PP: Estimation of person
  parameters for the 1,2,3,4-PL model and the GPCM. R package version
  0.6.3-11. https://github.com/jansteinfeld/PP

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {PP: Estimation of person parameters for the 1,2,3,4-PL model and the GPCM.},
    author = {Manuel Reif and Jan Steinfeld},
    year = {2021},
    note = {R package version 0.6.3-11},
    url = {https://github.com/jansteinfeld/PP},
  }