Description:
------------
PHP Function is_writable() sometimes reports false results.

Requirements: Linux - getuid() != geteuid()

After inspection of PHP source, I realized, that is_writable() internally uses syscall access(2). In turn,  access(2) man page (on Linux) says:

       The check is done with the calling process’s real UID and  GID,  rather
       than  with  the  effective  IDs  as is done when actually attempting an
       operation.  This is to allow set-user-ID programs to  easily  determine
       the invoking user’s authority.


Reproduce code:
---------------
<?
$file = "testfile.txt";

echo "GetUID: " . posix_getuid() . "/" . posix_geteuid() . "<br />";
echo "file is writable " .  (is_writable($file)?"TRUE":"FALSE") . "<br />";

fopen("testfile.txt", 'wt');
echo ("file created");
?>


Expected result:
----------------
Expected result:

either: 
file is writable TRUE
file created

or
file is writable FALSE
ERROR



Actual result:
--------------
GetUID: 933/10099
file is writable FALSE
file created