Although this section is called "Including files" never get tempted to use include() or include_once() functions. They produce just warning if included file does not exist which is not what we need. include() or include_once() are anachronism from the past of PHP when there was no separation between data, logic and presentation. In TF context when we say that we "include" file what we really mean is require_once(). Also don't use require() - you may get into huge problems because multiple references to one and the same file (like form_field.class.php).

Including files from hidden/inc/ using require_once($WSC->get_site_inc_dir().'...');>

Including files from Tangra Library (tangra2/tangra_lib/lib/) using require_once(TANGRA_MAIN_DIR.'...');