Description:
------------
is_writable() only returns one for files that are writable by the apache user, not by the current UID.

Apache is running as user/group nobody, but is setup with virtual hosts.  

Each virtual host uses its own user/group.  The current UID as returned by getmyuid() verifies that my PHP scripts are running as "me" and not as "nobody".  

test_file is permission 0644.

If I chmod test_file to user "me" is_writable() returns nothing.

If I chmod test_file to user "nobody" is_writable() returns 1.

PHP needs to check the current uid an use that to verify if something is writable, or it needs to defer to the operating system.  

I imagine this is mod_php specific.

Reproduce code:
---------------
httpd.conf :

User nobody
Group nobody

<VirtualHost ...>
  User me
  Group me
</VirtualHost>

$ touch test_file
$ chmod 0644 test_file
$ chown nobody test_file

<? print getmyuid() ?>
<? print is_writable('test_file') ?>
 514
 1

$ chown me test_file

<? print getmyuid() ?>
<? print is_writable('test_file') ?>
  514
  (nothing)

Expected result:
----------------
I expect is_writable() to return 1 if PHP fully has permission to write the file. 

As a result, I have to chmod sensitive data to 0666 on a shared server, which is bad.