Seeing as is_readable, is_writeable and is_executable all
work out the file permissions masks themselves, it might
be nice to have access to the posix function access(2) so that
file permissions in situations where mask comparision is
not sufficient (e.g. when using POSIX ACL's on the filesystem,
such as is available with SGI's XFS on Linux and Irix).

It should simply be a matter of (in ext/posix/posix.c

  #include <unistd.h>
  ....
  PHP_FE(posix_access, NULL)
  ....

  /* {{{ proto bool posix_access(string filename, int mode)
   check user's permissions for a file */
  PHP_FUNCTION(posix_access)
  {
    char *filename = NULL;
    int argc = ZEND_NUM_ARGS();
    int mode = 0;
    if (zend_parse_parameters(argc TSRMLS_CC, "s|l", &filename, &mode) == FAILURE)
      return;

    if (access(filename, mode) != 0) {
      php_error(E_WARNING,"posix_access: error checking access for %s: %s", filename, strerror(errno));
    } else {
      return 0;
    }
  }
 
As far as I can see, access() has always been a POSIX function.