__ FILE__ difference with $ _SERVER [SCRIPT_FILENAME '']: the absolute path to get the file

__FILE__ : get the absolute path of a file in the local directory (that is, which documents the implementation of this line of code, the absolute path of the file which it acquired )

__ SERVER $ [ 'SCRIPT_FILENAME']   : get the current url absolute path routing (that is to say, we Url, the visit is which file, it will get the absolute path of the file )