PHP 4.0 Beta Patch Level 1 fails to compile on HP-UX10.20. 
The failure occurs in the localtime_r function.
On HP-UX, this function is defined as follows (copied from /usr/include/time.h):

extern struct tm *localtime(const time_t *);   
extern int localtime_r(const time_t *, struct tm *);

There are some similar warnings that appear for gmtime_r function too. 
Similar problem appears in the following files:
          ext/standard/parsedate.y
          ext/standard/datetime.c
          main.c

I was able to compile when I used the solution in  Bug id #3392.