Description:
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RE Bug #36164 which describes the problem perfectly and to which I would have added this comment had I been permitted to:

If I'm not mistaken, the problem is a name clash between main/php_compat.h and /usr/include/tidy/tidy.h and I don't see why it is the tidy developers who are responsible for this.

Specifically, ext/tidy/tidy.c first includes main/php_config.php (via main/php.h -> main/php_compat.h),
which provides the definition

#define ulong unsigned long 

Then ext/tidy/tidy.c includes /usr/include/tidy/platform.h via /usr/include/tidy/tidy.h.  But platform.h has the lines

#if defined(HPUX_OS) || defined(CYGWIN_OS) || defined(MAC_OS) || defined(BSD_BASED_OS) || defined(_WIN32)
typedef unsigned long ulong;
#endif

At least for the specified OS's, which includes Mac OS, this combination of the #define in the php.h header with the typedef in the tidy.h header results in the nonsensical "typedef unsigned long unsigned long" which results in the reported (Bug #36164) compilation error.

IMHO, and with all due respect to tony2001, the tidy developers would be well within their rights to assert that they are not responsible for a name clash with PHP, even if the simplest work around is to incorporate a "!defined(ulong) && (...)" in the above excerpt from tidy.h.  Indeed, if PHP is using tidy, and is creating the name clash, is not PHP responsible for this?

Would it be possible to simply rearrange the #include's in tidy.c, so that tidy.h precedes php.h, and also surround the "#define ulong..." in php_config.h with an

#if !defined(__PLATFORM_H__) || !(defined(HPUX_OS) || defined(CYGWIN_OS) || defined(MAC_OS) || defined(BSD_BASED_OS) || defined(_WIN32))

as (I think) this would prevent the name clash?  Or perhaps there is an even better solution?