Specific input/output file name extensions aren't prescribed in phrep.

Include searching

For #include <basename> directives, a few file extensions are searched implicitly:

• .ph
• .php
• .inc
• .h
• .p

Output files

Proccessed -o output files usually are *.php scripts.

Input sources

While -i input files can use any extension.

• .src is recommended and preferred by the Makefile examples.
• .ph could also be used to signalize partial PHP scripts.
• .phpp is used by PHPP.
• .p.php is what preprocess.py recommends for input files.

Just decide on one, depending on legibility.

Other extensions

• .def includes are treated much like C header .h files. They're meant for longer #macro definition files. Both .def and .h files have their output suppressed per default - to avoid interpolating whitespace between #define lists.

• .ph is meant to mean PHP header, in contrast to C header .h includes.

• .src as default input file extension is advised for PHP-only projects. For mixed-language projects, a more schematic suffix scheme like .p.php ad .p.py might be more obvious.

• .inc is a PHP4-era idiom, but might make sense as indicator for code + partial html/content includes.