These warnings are not serious.
But my compiler (Sun Workshop Compiler) claims a lot.
I hope these are fixed.

fopen-wrappers.c:424 base64_encode()
  proto: const unsigned char *
  used: char *

functions/html.c:76
  unsigned char * = char *

functions/md5.c:71 PHP3_MD5Update
  proto: const unsigned char *
  used: char *

functions/string.c:588,589 strlen()
  proto: const char *
  used: unsigned char *

functions/base64.c:46,82,149,168 strlen()
  proto: const char *
  used: const unsigned char *, unsigned char *

functions/base64.c:146,165 base64_encode()
  proto: const unsigned char *
  used: char *

functions/base64.c:148
  char * = unsigned char *