I guess we could add also (beside other facts)
- about how to deal with it's argument, as it's not so straightforward: without prefix ./ or ../ it's not the directory of script, where php searches first, but in include_path, which is path defined in the php's options and when unsuccessful, then the engine searches in the directory of the current script (so be careful not to include different file than you wanted)
- in the file you include all the variables from before including are also avalible in the included script, all functiones and classes from a included file have global scope
- using include construct is not http request, so no absolute http://, no GET attributes in url
- about including either content to center or top and bottom part (I prefer this way) of page when constructing a dynamic webpage