A common convention is to name internal C header files as `*_int.h`.
Since a couple of these are actually installed, we add comments that
this is not supposed to happen, (a) to avoid installing further
internal headers, and (b) to pave the way to fix this in the next major
PHP version.
Somewhat special is php_gmp_int.h, where "int" is meant as abbreviation
for "interface".
Another common convention is appending `_priv` or `_private`, but since
there have not been any issues regarding these headers so far, we
refrain from adding respective comments to these headers.
Anyhow, it might be a good idea to introduce some common naming
convention for such internal/private headers.
Basically all constants are now declared via stubs. The rest of the constants are either deprecated (`SID` or `MHASH_*`) or out of interest (`__COMPILER_HALT_OFFSET__` and `PHP_CLI_PROCESS_TITLE`).
