Specific input/output file name extensions aren't prescribed in phrep.
#include <basename> directives, a few file extensions are searched
Proccessed -o output files usually are
While -i input files can use any extension.
.srcis recommended and preferred by the Makefile examples.
.phcould also be used to signalize partial PHP scripts.
.phppis used by PHPP.
.p.phpis what preprocess.py recommends for input files.
Just decide on one, depending on legibility.
.defincludes are treated much like C header
.hfiles. They're meant for longer #macro definition files. Both
.hfiles have their output suppressed per default - to avoid interpolating whitespace between #define lists.
.phis meant to mean PHP header, in contrast to C header
.srcas 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.
.incis a PHP4-era idiom, but might make sense as indicator for code + partial html/content includes.