PHP file loading mechanism in the end what is the directory

PHP file loading seemingly simple, in fact, the mechanism is not so simple, on the contrary, often encountered in the use of some puzzling problems, today, to say this mechanism

A sentence summary:

If the./or. /start, then the reference point is to access the file as a browser

If not, then it is the first to see the include_path, then see the current file, finally see the browser access to the file

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PHP file loading mechanism in the end what is the directory