To cite phtt in publications use:

Oualid Bada, Dominik Liebl (2014). The R Package phtt: Panel Data Analysis with Heterogeneous Time Trends. Journal of Statistical Software, 59(6), 1-34. URL http://www.jstatsoft.org/v59/i06/.

Corresponding BibTeX entry:

  @Article{,
    title = {The {R} Package {phtt}: Panel Data Analysis with
      Heterogeneous Time Trends},
    author = {Oualid Bada and Dominik Liebl},
    journal = {Journal of Statistical Software},
    year = {2014},
    volume = {59},
    number = {6},
    pages = {1--34},
    url = {http://www.jstatsoft.org/v59/i06/},
  }